A pedigree of one family with delayed sleep phase syndrome

Updates and confounding factors in delayed sleep-wake phase disorder

Delayed sleep-wake phase disorder (DSWPD) is a circadian rhythm sleep disorder characterised by a delay in the main sleep period, with patients experiencing difficulty getting to sleep and waking up at socially appropriate times. This often causes insomnia and compromised sleep, results in impairment to daytime function and is associated with a range of comorbidities. Besides interventions aimed at ameliorating symptoms, there is good evidence supporting successful phase advancement with bright light therapy or melatonin administration. However, no treatment to date addresses the tendency to phase delay, which is a common factor amongst the various contributing causes of DSWPD. Circadian phase markers such as core body temperature and circulating melatonin typically correlate well with sleep timing in healthy patients, but numerous variations exist in DSWPD patients that can make these unpredictable for use in diagnostics. There is also increasing evidence that, on top of problems with the circadian cycle, sleep homeostatic processes actually differ in DSWPD patients compared to controls. This naturally has ramifications for management but also for the current approach to the pathogenesis itself in which DSWPD is considered a purely circadian disorder. This review collates what is known on the causes and treatments of DSWPD, addresses the pitfalls in diagnosis and discusses the implications of current data on modified sleep homeostasis, making clinical recommendations and directing future research.

Circadian Rhythm Sleep Disorders: Genetics, Mechanisms, and Adverse Effects on Health

Nearly all living organisms, from cyanobacteria to humans, have an internal circadian oscillation with a periodicity of approximately 24 h. In mammals, circadian rhythms regulate diverse physiological processes including the body temperature, energy metabolism, immunity, hormone secretion, and daily sleep-wake cycle. Sleep is tightly regulated by circadian rhythms, whereas a misalignment between the circadian rhythms and external environment may lead to circadian rhythm sleep disorders (CRSD). CRSD includes four main kinds of disorders: the advanced sleep-wake phase disorder (ASPD), the delayed sleep-wake phase disorder (DSPD), the irregular sleep-wake rhythm disorder and the non-24-h sleep-wake rhythm disorder. Recent studies have begun to shed light on the genetic basis of CRSD. Deciphering the genetic codes for ASPD and DSPD has so far been more successful than the other CRSDs, which allow for the development of animal models and understanding of the pathological mechanisms for these disorders. And studies from humans or animal models implicate CRSDs are associated with adverse health consequences, such as cancer and mental disorders. In this review, we will summarize the recent advances in the genetics, underlying mechanisms and the adverse effects on health of ASPD and DSPD.

Transcriptional Control of Circadian Rhythms and Metabolism: A Matter of Time and Space

All biological processes, living organisms, and ecosystems have evolved with the Sun that confers a 24-hour periodicity to life on Earth. Circadian rhythms arose from evolutionary needs to maximize daily organismal fitness by enabling organisms to mount anticipatory and adaptive responses to recurrent light-dark cycles and associated environmental changes. The clock is a conserved feature in nearly all forms of life, ranging from prokaryotes to virtually every cell of multicellular eukaryotes. The mammalian clock comprises transcription factors interlocked in negative feedback loops, which generate circadian expression of genes that coordinate rhythmic physiology. In this review, we highlight previous and recent studies that have advanced our understanding of the transcriptional architecture of the mammalian clock, with a specific focus on epigenetic mechanisms, transcriptomics, and 3-dimensional chromatin architecture. In addition, we discuss reciprocal ways in which the clock and metabolism regulate each other to generate metabolic rhythms. We also highlight implications of circadian biology in human health, ranging from genetic and environment disruptions of the clock to novel therapeutic opportunities for circadian medicine. Finally, we explore remaining fundamental questions and future challenges to advancing the field forward.

The Epidemiology of Morningness/Eveningness: Influence of Age, Gender, Ethnicity, and Socioeconomic Factors in Adults (30-49 Years)

The Horne and Ostberg Morningness/Eveningness Questionnaire (MEQ) is widely used to differentiate between morning and evening types, but there is very little epidemiological evidence about the distribution of MEQ chronotypes in the general population. The purpose of the present study was to simultaneously investigate the influence of demographic, socioeconomic, and work factors on the distribution of morningness/eveningness. A New Zealand version of the MEQ was mailed to 5000 New Zealand adults, ages 30 to 49 years, who were randomly selected from the electoral rolls (55.7% response rate). A total of 2526 questionnaires were included in the analyses. According to the Horne and Ostberg classification, 49.8% of the total population was classified as morning type compared to 5.6% having an evening-type preference. However, using new cutoffs for middle-aged working adults described by Taillard et al. (2004), 24.7% of the population was morning type and 26.4% was evening type. After controlling for ethnicity, gender, and socioeconomic deprivation, participants ages 30 to 34 years were more likely to be definitely evening type (odds ratio [OR] = 1.59, p < 0.05) and less likely to be morning type (moderately morning type, OR = 0.59, p < 0.01, or definitely morning type, OR = 0.59, p < 0.05) compared to those ages 45 to 49 years. Work schedules were also important predictors of chronotype, with night workers more likely to be definitely evening type (OR = 1.49, p = 0.05) and the unemployed less likely to be moderately morning type (OR = 0.64, p < 0.05) compared to other workers. Evening types were 2.5 times more likely to report that their general health was only poor or fair compared to morning types ( p < 0.01). This study confirms that the original criteria of Horne and Ostberg (1976) are not useful for classifying chronotypes in a middle-aged population. The authors conclude that morningness/eveningness preference is largely independent of ethnicity, gender, and socioeconomic position, indicating that it is a stable characteristic that may be better explained by endogenous factors.

The ticking clock of Cayo Santiago macaques and its implications for understanding human circadian rhythm disorders

The circadian clock disorders in humans remain poorly understood. However, their impact on the development and progression of major human conditions, from cancer to insomnia, metabolic or mental illness becomes increasingly apparent. Addressing human circadian disorders in animal models is, in part, complicated by inverse temporal relationship between the core clock and specific physiological or behavioral processes in diurnal and nocturnal animals. Major advantages of a macaque model for translational circadian research, as a diurnal vertebrate phylogenetically close to humans, are further emphasized by the discovery of the first familial circadian disorder in non‐human primates among the rhesus monkeys originating from Cayo Santiago. The remarkable similarity of their pathological phenotypes to human Delayed Sleep Phase Disorder (DSPD), high penetrance of the disorder within one branch of the colony and the large number of animals available provide outstanding opportunities for studying the mechanisms of circadian disorders, their impact on other pathological conditions, and for the development of novel and effective treatment strategies. Am. J. Primatol. 78:117–126, 2016. © 2016 The Authors. American Journal of Primatology published by Wiley Periodicals, Inc.

Chronobiology and insomnia: pathophysiology and treatment of circadian rhythm sleep disorders

This review summarizes current knowledge of sleep disorders with a chronobiological basis, including: delayed sleep phase syndrome, advanced sleep phase syndrome, non24 h sleep-wake syndrome and irregular sleep-wake pattern disorder. These circadian rhythm sleep disorders are characterized by a misalignment between the timing of the sleep period with respect to the day-night cycle and as a consequence of patients attempting to maintain 'normal' social hours, reduced sleep quality. In addition to the specific circadian rhythm sleep disorders, this review will also examine current drug (e.g., hypnotics and melatonin) and nondrug (e.g., bright light therapy and chronotherapy) treatments, the overlap with psychophysiological insomnia and future directions.

Classification and epidemiology of sleep disorders

Sleep disturbances are common in children and adolescents but still remain underrecognized and undertreated. Several classification systems of sleep disorders are available, including some newer attempts to develop more specific nosologic categories that reflect developmental aspects of sleep. The prevalence of sleep disorders has been studied across various samples of healthy, normal children and in children with special medical and neurodevelopmental needs. Sleep disorders are more frequently seen in children and adolescents with psychiatric disorders, making it very important for mental health professionals to be aware of sleep problems and to address them in the context of psychiatric comorbidities.

Sleep during a regular week night: a twin-sibling study

Previous genetic investigations of variation in normal sleep have focused on measures that describe sleep over longer periods of time. We undertook a study with the aim of evaluating whether heritability can be found in single-night sleep traits. A classical twin study design of monozygotic and dizygotic twins, enriched with siblings of twins was employed. The study included adult twin pairs and their siblings (N = 813 subjects from 342 families). A subsample of 66 individuals participated twice. For a single night, bedtime, awakening time and subjective sleep quality were assessed using a diary. The diary also assessed smoking, alcohol and coffee consumption, and the subjective evaluation of stress. Resemblance between family members was used to estimate the heritability of bedtime, awakening time, sleep problems and sleep quality as a function of sex. Most sleep measures showed familial clustering, but results differed for men and women. Heritability for bedtime and sleep problems was seen in women; and for awakening time in men. We conclude that heritability can be demonstrated for bedtime and subjective evaluation of even a single night of sleep. The contribution of the genetic make-up is sex specific. In women variance in awakening time is so affected by environmental circumstances, that the genetic contribution to the variance becomes negligible. In contrast, for males, variance in the evening bedtime is so affected by environmental circumstances, that the genetic contribution to the variance becomes negligible.

Phenotypic effects of genetic variability in human clock genes on circadian and sleep parameters

Circadian rhythms and sleep are two separate but intimately related processes. Circadian rhythms are generated through the precisely controlled, cyclic expression of a number of genes designated clock genes. Genetic variability in these genes has been associated with a number of phenotypic differences in circadian as well as sleep parameters, both in mouse models and in humans. Diurnal preferences as determined by the selfreported Horne-Ostberg (HO) questionnaire, has been associated with polymorphisms in the human genes CLOCK, PER1, PER2 and PER3. Circadian rhythm-related sleep disorders have also been associated with mutations and polymorphisms in clock genes, with the advanced type cosegrating in an autosomal dominant inheritance pattern with mutations in the genes PER2 and CSNK1D, and the delayed type associating without discernible Mendelian inheritance with polymorphisms in CLOCK and PER3. Several mouse models of clock gene null alleles have been demonstrated to have affected sleep homeostasis. Recent findings have shown that the variable number tandem polymorphism in PER3, previously linked to diurnal preference, has profound effects on sleep homeostasis and cognitive performance following sleep loss, confirming the close association between the processes of circadian rhythms and sleep at the genetic level.

Moléculas que marcam o tempo: implicações para os fenótipos circadianos

OBJETIVO: Revisar resumidamente a literatura dos últimos 36 anos de pesquisa em cronobiologia molecular a fim de informar aos profissionais de saúde os avanços obtidos nesta área e os potenciais para aplicação na clínica médica. MÉTODO: Buscas na literatura foram realizadas utilizando as bases de dados PubMed e Scopus usando como palavras-chave "clock genes, circadian rhythms, diurnal preference, delayed sleep phase syndrome, advanced sleep phase syndrome, photoperiod and mood disorder". DISCUSSÃO: Atualmente, o mecanismo molecular da regulação da ritmicidade circadiana é compreendido em grande detalhe. Muitos estudos publicados mostram associações de polimorfismos nos genes relógio com transtornos do ritmo circadiano e com transtornos do humor. CONCLUSÕES: De maneira geral, o progresso obtido na área de cronobiologia molecular traz um melhor entendimento da regulação do sistema de temporização biológico. O desenvolvimento de estudos nesta área tem o potencial de ser aplicável ao tratamento dos transtornos dos ritmos circadianos e certos transtornos do humor, além de prevenir riscos à saúde causados por viagens intercontinentais (Jet Lag) e por trabalhos noturnos e por turnos.

The genetics of mammalian circadian order and disorder: implications for physiology and disease

Circadian cycles affect a variety of physiological processes, and disruptions of normal circadian biology therefore have the potential to influence a range of disease-related pathways. The genetic basis of circadian rhythms is well studied in model organisms and, more recently, studies of the genetic basis of circadian disorders has confirmed the conservation of key players in circadian biology from invertebrates to humans. In addition, important advances have been made in understanding how these molecules influence physiological functions in tissues throughout the body. Together, these studies set the scene for applying our knowledge of circadian biology to the understanding and treatment of a range of human diseases, including cancer and metabolic and behavioural disorders.

The impairment of pediatric bipolar sleep: hypotheses regarding a core defect and phenotype-specific sleep disturbances

BACKGROUND

The nature of the sleep disturbances associated with different phenotypes of pediatric bipolar disorder is unknown. Most manic children exhibit delayed sleep onset, but only a minority display decreased need for sleep. The DSM-IV manic sleep criterion is inadequate.

METHOD

All published studies of the sleep characteristics of bipolar children and adolescents are reviewed. Relevant studies of pediatric unipolar depressed subjects, circadian variation of bipolar and sleep variables, and circadian neurobiology are also reviewed. This information forms the basis of hypotheses regarding the core defect of pediatric bipolar sleep and phenotype-specific sleep disturbances of bipolar children and adolescents.

LIMITATIONS

The extant research literature is extremely limited. Interpretation of bipolar sleep is confounded by day-to-day variation of bipolar symptoms and sleep parameters, the presence of comorbid conditions, and environmental and psychosocial factors.

CONCLUSIONS

The core defect of pediatric bipolar sleep is hypothesized to be a significant delay of the circadian sleep-wake cycle, a form of the delayed sleep-phase syndrome. Children and adolescents with part-day manic cycles and chronic mixed conditions typically will manifest delayed sleep onset, but not decreased need for sleep. Pediatric individuals with days-long manic cycles or chronic mania typically will manifest decreased need for sleep, produced by interaction between the sleep-onset phase delay and bedtime and early morning manic psychomotor acceleration. The sleep-onset phase delay, when expressed, is hypothesized to be a trait marker of bipolar spectrum illness. Revision of the DSM-IV manic sleep criterion is necessary.

Delayed sleep phase cases and controls

BACKGROUND

Delayed sleep phase disorder (DSPD) is a condition in which patients have difficulty falling asleep before the early morning hours and commonly have trouble awakening before late morning or even early afternoon. Several studies have suggested that variations in habitual bedtime are 40-50% heritable.

METHODS

We recruited a case series of 205 participants, along with 221 controls (DSPD-C) with normal sleep, roughly matched for age, gender, and ancestry. A representative sample of San Diego adults recruited some years before was already available to confirm the control group. Both DSPD and DSPD-C provided blood or saliva samples for DNA and completed extensive questionnaires about sleep habits, sleep history, family history, sleep quality, morningness-eveningness traits, depression, mania, and seasonality of symptoms. The DSPD group wore wrist actigraphs for a median of 13.2 days. The representative sample collected previously had undergone actigraphic recordings, from which 48 hours of data were generally available.

RESULTS

The DSPD and DSPD-C samples showed almost no overlap on morningness-eveningness scores. DSPD cases went to bed and arose about 3 hours later than the DSPD-C and the representative sample. DSPD cases reported more difficulties with sleep, poorer sleep quality, and more depression, but there was no significant difference in a history of mania. DSPD cases reported more family history of late bedtimes, but female DSPD reported that their fathers' bedtimes were later than the fathers of male DSPD.

CONCLUSION

These results indicate a DSPD phenotype is familial and associated with unipolar depression.

Circadian rhythm sleep disorders: part II, advanced sleep phase disorder, delayed sleep phase disorder, free-running disorder, and irregular sleep-wake rhythm. An American Academy of Sleep Medicine review

OBJECTIVE

This the second of two articles reviewing the scientific literature on the evaluation and treatment of circadian rhythm sleep disorders (CRSDs), employing the methodology of evidence-based medicine. We herein report on the accumulated evidence regarding the evaluation and treatment of Advamced Sleep Phase Disorder (ASPD), Delayed Sleep Phase Disorder (DSPD), Free-Running Disorder (FRD) and Irregular Sleep-Wake Rhythm ISWR).

METHODS

A set of specific questions relevant to clinical practice were formulated, a systematic literature search was performed, and relevant articles were abstracted and graded.

RESULTS

A substantial body of literature has accumulated that provides a rational basis the evaluation and treatment of CRSDs. Physiological assessment has involved determination of circadian phase using core body temperature and the timing of melatonin secretion. Behavioral assessment has involved sleep logs, actigraphy and the Morningness-Eveningness Questionnaire (MEQ). Treatment interventions fall into three broad categories: 1) prescribed sleep scheduling, 2) circadian phase shifting ("resetting the clock"), and 3) symptomatic treatment using hypnotic and stimulant medications.

CONCLUSION

Circadian rhythm science has also pointed the way to rational interventions for CRSDs and these treatments have been introduced into the practice of sleep medicine with varying degrees of success. More translational research is needed using subjects who meet current diagnostic criteria.

From circadian rhythms to clock genes in depression

Depression, particularly unipolar depression, has often been linked to circadian (i.e. approximately 24-h) rhythm abnormalities. The observation that many diverse rhythms are disrupted in depressed patients indicates that such disturbances are not unique to specific rhythms, but instead are of a more central origin (i.e. involve the central circadian pacemaker and/or the molecular circadian clock core machinery). One rhythm that is often disrupted in depression is the sleep-wake cycle - a disruption that, in turn, might lead to other rhythm disturbances. Thus, there are two general ways in which disrupted circadian rhythms could lead to depression: (i) disorganization within the circadian system could itself lead to neurobiological dysfunction and (ii) a circadian disturbance of the normal sleep-wake cycle could facilitate or exacerbate the depressed state. The recent discovery of the molecular clock responsible for the generation of circadian rhythms provides novel mechanistic insights into how rhythm abnormalities might lead to disrupted behavioural states, and offers new therapeutic avenues for the treatment of the timing abnormalities that might underlie depression. The finding that the molecular circadian clock is present in many cells in the central nervous system and regulates the timing of the expression of at least 10% of the transcripts in many tissues emphasizes how circadian dysfunction could have drastic consequences for normal physiological function in the brain.

From circadian clock gene expression to pathologies

In most organisms, circadian rhythms are generated by a molecular clockwork involving so-called clock genes. These circadian clock genes participate in regulatory feedback loops, in which proteins regulate their own expression. The outcome is that ribonucleic acids (RNAs) and proteins produced from many of these genes oscillate with a circadian rhythm. Here, we describe the regulation of clock genes and proteins, as deduced from work in rodents. Furthermore, we summarize the work done on human clock genes and their expression in peripheral tissues. Importantly, the research reviewed here points to an implication of clock gene defects in circadian rhythm disorders, including the advanced and delayed sleep phase disorders. Moreover, circadian clock gene dysfunction is likely to be of importance in the development of cancer as well as various other diseases.

A clinical approach to circadian rhythm sleep disorders

Circadian rhythm sleep disorders are characterized by complaints of insomnia and excessive sleepiness that are primarily due to alterations in the internal circadian timing system or a misalignment between the timing of sleep and the 24-h social and physical environment. In addition to physiological and environmental factors, maladaptive behaviors often play an important role in the development of many of the circadian rhythm sleep disorders. This review will focus on the clinical approach to the diagnosis and management of the various circadian rhythm sleep disorders, including delayed sleep phase disorder, advanced sleep phase disorder, non-entrained type, irregular sleep-wake rhythm, shift work sleep disorder and jet lag disorder. Diagnostic tools such as sleep diaries and wrist activity monitoring are often useful in confirming the diagnosis. Because behavioral and environmental factors often are involved in the development of these conditions, a multimodal approach is usually necessary. Interventions include sleep hygiene education, timed exposure to bright light as well as avoidance of bright light at the wrong time of the day and pharmacologic approaches, such as melatonin. However, it should be noted that the use of melatonin is not an FDA-approved indication for the treatment of circadian rhythm sleep disorders.

Circadian rhythm sleep disorders and phototherapy

Circadian rhythm sleep disorders are characterized by a desynchronization between the timing of the intrinsic circadian clock and the extrinsic light-dark and social/activity cycles resulting in symptoms of excessive sleepiness and insomnia. This article explores the six recognized circadian rhythm sleep disorders: delayed sleep phase syndrome, advanced sleep phase syndrome, non-24-hour sleep-wake syndrome, irregular sleep-wake pattern, shift work sleep syndrome, and time zone change syndrome. Additionally discussed are the therapeutic roles of synchronizing agents, such as light and melatonin.

Behavioral and psychiatric consequences of sleep-wake schedule disorders

Circadian rhythm sleep disorders (CRSDs) arise when an individual's sleep-wake rhythm mismatches the environmental 24-h schedule. Physiological data and genetic studies in patients with CRSDs suggest that these disorders result from abnormal functioning of the circadian timing system. Diagnosis involves recognition of the characteristics of CRSDs, which can be achieved by clinical interview and actigraphic monitoring of rest-activity patterns. Bright-light therapy and melatonin administration have proved to be the most effective treatment modalities of CRSDs. In psychiatric practice, CRSDs can be encountered on various occasions. Some evidence indicates that a deviant sleep-wake schedule might be a predisposing factor to personality disorders. CRSDs can emerge as an iatrogenic effect of certain psychoactive drugs, such as haloperidol and fluvoxamine. It is not uncommon that the daytime functional difficulties that accompany CRSDs are misinterpreted as symptoms of psychopathology. Recognition and awareness of these disorders should prevent years of erroneous diagnosis and treatment in these patients.

Diagnosis and treatment of sleep disorders in older adults

Among the most common complaints of older adults are difficulty initiating or maintaining sleep. These problems result in insufficient sleep at night, which then results in an increased risk of falls, difficulty with concentration and memory, and overall decreased quality of life. Difficulties sleeping, however, are not an inevitable part of aging. Rather, these sleep complaints are often secondary to medical and psychiatric illness, the medications used to treat these illnesses, circadian rhythm changes, or other sleep disorders. The task for the geriatric psychiatrist is to identify the causes of these complaints and then initiate appropriate treatment.

Prevalence and Comorbidity of Insomnia and Effect on Functioning in Elderly Populations

A good night's sleep is often more elusive as we age, because the prevalence of insomnia in older people is high. Insufficient sleep can have important effects on daytime function by increasing the need to nap, reducing cognitive ability including attention and memory, slowing response time, adversely affecting relationships with friends and family, and contributing to a general sense of being unwell. However, rather than aging per se, circadian rhythm shifts, primary sleep disorders, comorbid medical/psychiatric illnesses, and medication use cause sleep difficulties in older people, which psychosocial factors may also affect. Clinicians should ask elderly patients about satisfaction with sleep. Any sleep complaints warrant careful evaluation of contributing factors and appropriate treatment.

Circadian Rhythm Sleep Disorders

Individuals who have circadian rhythm sleep disorders present with symptoms of insomnia or excessive sleepiness and complain of an inability to sleep at their desired time. Although the primary etiology of these disorders is a misalignment between the endogenous circadian clock and the external environment, social and behavioral factors can also play important roles in perpetuating or exacerbating these disorders. Currently, the management of circadian rhythm disorders is limited to the use of bright light and melatonin to realign the circadian clock with the desired sleep time.However, as the understanding of the physiologic and genetic basis of sleep and circadian rhythm regulation advances, even more practical and effective treatments should become available.

The 3111T/C polymorphism of hClock is associated with evening preference and delayed sleep timing in a Japanese population sample

Sleep timing is influenced by the circadian system. Morningness-eveningness (ME) preference in humans is affected by the free-running period, which is determined by circadian clock-relevant genes. In this study, we investigated association between the 3111T/C polymorphism in the 3'-flanking region of hClock (Homo sapiens Clock homolog) and ME preference in 421 Japanese subjects. The Horne-Ostberg ME questionnaire (MEQ) scores showed normal distribution, with mean score of 51.2 +/- 1.4 (range, 25-73), and scores were positively correlated with sleep onset time (r = 0.541, P < 0.001) and wake time (r = 0.513, P < 0.001). MEQ scores were significantly lower in subjects with 3111C/C (n = 12) than in subjects with 3111T/C (n = 106, P < 0.001) or 3111T/T (n = 303, P < 0.001), suggesting a stronger eveningness preference in 3111C/C homozygotes. This group also showed significantly delayed sleep onset (P < 0.001), shorter sleep time (P < 0.001), and greater daytime sleepiness (P < 0.001) in comparison to parameters in the subjects with the 3111T allele. There was no significant difference in any of these parameters between the 3111C/T and 3111T/T genotypes. The influence of the 3111T/C polymorphism on ME preferences in Caucasian populations remains controversial. The present findings in a Japanese population sample, which should have a relatively low risk of population stratification effects, suggest the significance of the association of the 3111C/C allele of hClock with evening preference.

A missense variation in human casein kinase I epsilon gene that induces functional alteration and shows an inverse association with circadian rhythm sleep disorders

Recent studies have shown that functional variations in clock genes, which generate circadian rhythms through interactive positive/negative feedback loops, contribute to the development of circadian rhythm sleep disorders in humans. Another potential candidate for rhythm disorder susceptibility is casein kinase I epsilon (CKIepsilon), which phosphorylates clock proteins and plays a pivotal role in the circadian clock. To determine whether variations in CKIepsilon induce vulnerability to human circadian rhythm sleep disorders, such as delayed sleep phase syndrome (DSPS) and non-24-h sleep-wake syndrome (N-24), we analyzed all of the coding exons of the human CKIepsilon gene. One of the variants identified encoded an amino-acid substitution S408N, eliminating one of the putative autophosphorylation sites in the carboxyl-terminal extension of CKIepsilon. The N408 allele was less common in both DSPS (p = 0.028) and N-24 patients (p = 0.035) compared to controls. When DSPS and N-24 subjects were combined, based on an a priori prediction of a common mechanism underlying both DSPS and N-24, the inverse association between the N408 allele and rhythm disorders was highly significant (p = 0.0067, odds ratio = 0.42, 95% confidence interval: 0.22-0.79). In vitro kinase assay revealed that CKIepsilon with the S408N variation was approximately 1.8-fold more active than wild-type CKIepsilon. These results indicate that the N408 allele in CKIepsilon plays a protective role in the development of DSPS and N-24 through alteration of the enzyme activity.

A molecular perspective of human circadian rhythm disorders

A large number of physiological variables display 24-h or circadian rhythms. Genes dedicated to the generation and regulation of physiological circadian rhythms have now been identified in several species, including humans. These clock genes are involved in transcriptional regulatory feedback loops. The mutation of these genes in animals leads to abnormal rhythms or even to arrhythmicity in constant conditions. In this view, and given the similarities between the circadian system of humans and rodents, it is expected that mutations of clock genes in humans may give rise to health problems, in particular sleep and mood disorders. Here we first review the present knowledge of molecular mechanisms underlying circadian rhythmicity, and we then revisit human circadian rhythm syndromes in light of the molecular data.

Human clock genes

Rhythmic variations in physiological and behavioural processes are mediated by both endogenous and exogenous factors. Endogenous factors include self-sustaining biological pacemakers or clocks which in the absence of strong external influences self-sustain periodic rhythms in such diverse physiological and psychological processes as core body temperature, food intake, cognitive performance and mood. Clocks with endogenous periods near or at 24 h (called circadian clocks from the Latin, circa dies, meaning about one day) have been documented from prokaryotes to single cell eukaryotes to multi-cellular, complex animals such as flies, rodents and humans. Over the past few years, a revolution in the understanding of the molecular basis of these clocks has led to the identification of a number of core clock genes and their proteins, and the development of elegant feedback models to explain the molecular gears of circadian clocks. At least eight human orthologs of mouse core clock genes have been identified, and polymorphisms in two of these, hClock and hPer2, have been implicated in human sleep disorders. Remarkably, knowledge of these core clock genes and the development of sophisticated reporter systems to monitor clock gene promoter activity have led to the astonishing observation that our body is actually composed of millions of cellular clocks and oscillators whose co-ordinated activity gives rise to pronounced daily, monthly, and seasonal rhythms in physiology and behaviour. An idea that is gaining favour is that our physical and mental well-being is probably determined by the appropriate phasing of these millions of cellular clocks with recurring, meaningful events in the environment.

Association of structural polymorphisms in the human period3 gene with delayed sleep phase syndrome

Recent progress in biological clock research has facilitated genetic analysis of circadian rhythm sleep disorders, such as delayed sleep phase syndrome (DSPS) and non-24-h sleep-wake syndrome (N-24). We analyzed the human period3 (hPer3) gene, one of the human homologs of the Drosophila clock-gene period (Per), as a possible candidate for rhythm disorder susceptibility. All of the coding exons in the hPer3 gene were screened for polymorphisms by a PCR-based strategy using genomic DNA samples from sleep disorder patients and control subjects. We identified six sequence variations with amino acid changes, of which five were common and predicted four haplotypes of the hPer3 gene. One of the haplotypes was significantly associated with DSPS (Bonferroni's corrected P = 0.037; odds ratio = 7.79; 95% CI 1.59-38.3) in our study population. Our results suggest that structural polymorphisms in the hPer3 gene may be implicated in the pathogenesis of DSPS.