Dynamic resetting of the human circadian pacemaker by intermittent bright light

Melatonin and its relevance to jet lag

Jet lag is a disorder in which body rhythms are out of phase with the environment because of rapid travel across time zones. Although it often produces minor symptoms it can cause serious problems in those who need to make rapid critical decisions including airline pilots and business travelers. In this article the authors review basic knowledge underlying the body clock, the suprachiasmatic nucleus (SCN) of the hypothalamus, and the manner in which it regulates the sleep/wake cycle. The regulation of melatonin by the SCN is described together with the role of the melatonin receptors which are integral to its function as the major hormonal output of the body clock. Several factors are known that help prevent and treat jet lag, including ensuring adequate sleep, appropriate timing of exposure to bright light and treatment with melatonin. Because travel can cross a variable number of time zones and in two different directions, recommendations for treatment are given that correspond with these different types of travel. In addition to use of bright light and melatonin, other factors including timed exercise, timed and selective diets and social stimuli deserve study as potential treatments. Moreover, new melatonin agonists are currently under investigation for treatment of jet lag.

Sleep and circadian rhythms in humans

During the past 50 years, converging evidence reveals that the fundamental properties of the human circadian system are shared in common with those of other organisms. Concurrent data from multiple physiological rhythms in humans revealed that under some conditions, rhythms oscillated at different periods within the same individuals and led to the conclusion 30 years ago that the human circadian system was composed of multiple oscillators organized hierarchically; this inference has recently been confirmed using molecular techniques in species ranging from unicellular marine organisms to mammals. Although humans were once thought to be insensitive to the resetting effects of light, light is now recognized as the principal circadian synchronizer in humans, capable of eliciting weak (Type 1) or strong (Type 0) resetting, depending on stimulus strength and timing. Realization that circadian photoreception could be maintained in the absence of sight was first recognized in blind humans, as was the property of adaptation of the sensitivity of circadian photoreception to prior light history. In sighted humans, the intrinsic circadian period is very tightly distributed around approximately 24.2 hours and exhibits aftereffects of prior entrainment. Phase angle of entrainment is dependent on circadian period, at least in young adults. Circadian pacemakers in humans drive daily variations in many physiologic and behavioral variables, including circadian rhythms in alertness and sleep propensity. Under entrained conditions, these rhythms interact with homeostatic regulation of the sleep/wake cycle to determine the ability to sustain vigilance during the day and to sleep at night. Quantitative understanding of the fundamental properties of the multioscillator circadian system in humans and their interaction with sleep/wake homeostasis has many applications to health and disease, including the development of treatments for circadian rhythm and sleep disorders.

Bright light exposure during acute tryptophan depletion prevents a lowering of mood in mildly seasonal women

We investigated the influence of bright light exposure on the mood-lowering effect of acute tryptophan depletion (ATD). Mildly seasonal healthy young women without a personal or family history of psychiatric disorders remained in either dim or bright light during two test days. Tryptophan-deficient and nutritionally balanced amino acid mixtures were administered in counterbalanced order. Mood state was assessed using the Profile of Mood States (POMS) and Visual Analogue Scales (VAS). In dim light, ATD decreased POMS scores across most subscales, indicating a worsening of mood. In bright light, mood was unaffected by ATD. Thus, bright light blocked the worsening of mood caused by ATD. This was also observed on the positive mood VAS. These results indicate a direct, immediate interaction between bright light and serotonin function. Bright light might help protect against ATD-induced mood change by increasing serotonin above the threshold level below which there is a lowering of mood.

Decreased sensitivity to phase-delaying effects of moderate intensity light in older subjects

Aging is associated with a change in the relationship between the timing of sleep and circadian rhythms, such that the rhythms occur later with respect to sleep than in younger adults. To investigate whether a difference in the phase-delaying response to evening light contributes to this, we conducted a 9-day inpatient study in 10 healthy older (> or =65 y.o.) subjects. We assessed circadian phase in a constant routine, exposed each subject to a 6.5h broad-spectrum light stimulus beginning in the early biological night, and reassessed circadian phase. The stimuli spanned a range from very dim (approximately 2 lx) to very bright (approximately 8000 lx) indoor light. We found a significant dose-response relationship between illuminance and the phase shift of the melatonin rhythm, with evidence that sensitivity, but not the maximal response to light, differed from that of younger adults. These findings suggest an age-related reduction in the phase-delaying response to moderate light levels. However, our findings alone do not explain the altered phase relationship between sleep and circadian rhythms associated with aging.

Absence of apparent circadian rhythms of gonadotropins and free alpha-subunit in postmenopausal women: evidence for distinct regulation relative to other hormonal rhythms

Aging is associated with a decrease in gonadotropin levels in postmenopausal women (PMW) and is also associated with alterations in a number of circadian rhythms. The goals of this study were to determine the presence of circadian rhythms of gonadotropins and glycoprotein free alpha-subunit (FAS) in young and old PMW. Healthy, euthyroid PMW, ages 45 to 55 years (n = 11) and 70 to 80 years (n = 11), were admitted in the morning to start a 24-h constant routine of light, temperature, position, and activity. Subjects remained awake and semirecumbent for the duration of the study and were fed hourly snacks, and activity was monitored continuously. Blood was sampled every 5 min for two 8-h periods corresponding to the estimated acrophase and nadir of the temperature rhythm. Luteinizing hormone (LH) and FAS were measured in all samples and follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH), and cortisol in 20-min serum pools. Mean LH (p < 0.001), FSH (p < 0.002), and FAS (p < 0.002) were lower in older compared with younger PMW. Day/night differences in cortisol and TSH (p < 0.001) were present in all subjects. However, there were no day/night differences in LH in younger or older PMW or in FSH in younger or older PMW. There were no day/night differences in mean FAS in younger or older PMW or in FAS pulse frequency or amplitude. Thus, in controlled studies in which differences in cortisol and TSH were demonstrated, there were no day/night differences in LH, FSH, or FAS in PMW. These studies suggest that despite evidence of intact circadian rhythms of cortisol and TSH, gonadotropin secretion does not appear to follow a circadian pattern in PMW. Thus, the age-related decline in gonadotropin secretion in PMW is not associated with a dampening of circadian rhythmicity. The absence of day/night differences in FAS suggests that GnRH plays a more prominent role in FAS regulation than does thyrotropin-releasing hormone in PMW.

Intrinsic period and light intensity determine the phase relationship between melatonin and sleep in humans

The internal circadian clock and sleep-wake homeostasis regulate the timing of human brain function, physiology, and behavior so that wakefulness and its associated functions are optimal during the solar day and that sleep and its related functions are optimal at night. The maintenance of a normal phase relationship between the internal circadian clock, sleep-wake homeostasis, and the light-dark cycle is crucial for optimal neurobehavioral and physiological function. Here, the authors show that the phase relationship between these factors-the phase angle of entrainment (psi)-is strongly determined by the intrinsic period (tau) of the master circadian clock and the strength of the circadian synchronizer. Melatonin was used as a marker of internal biological time, and circadian period was estimated during a forced desynchrony protocol. The authors observed relationships between the phase angle of entrainment and intrinsic period after exposure to scheduled habitual wakefulness-sleep light-dark cycle conditions inside and outside of the laboratory. Individuals with shorter circadian periods initiated sleep and awakened at a later biological time than did individuals with longer circadian periods. The authors also observed that light exposure history influenced the phase angle of entrainment such that phase angle was shorter following exposure to a moderate bright light (approximately 450 lux)-dark/wakefulness-sleep schedule for 5 days than exposure to the equivalent of an indoor daytime light (approximately 150 lux)-dark/wakefulness-sleep schedule for 2 days. These findings demonstrate that neurobiological and environmental factors interact to regulate the phase angle of entrainment in humans. This finding has important implications for understanding physiological organization by the brain's master circadian clock and may have implications for understanding mechanisms underlying circadian sleep disorders.

Advancing circadian rhythms before eastward flight: a strategy to prevent or reduce jet lag

STUDY OBJECTIVES

To develop a practical pre-eastward flight treatment to advance circadian rhythms as much as possible but not misalign them with sleep.

DESIGN

One group had their sleep schedule advanced by 1 hour per day and another by 2 hours per day.

SETTING

Baseline at home, treatment in lab.

PARTICIPANTS

Young healthy adults (11 men, 15 women) between the ages of 22 and 36 years.

INTERVENTIONS

Three days of a gradually advancing sleep schedule (1 or 2 hours per day) plus intermittent morning bright light (one-half hour approximately 5000 lux, one-half hour of <60 lux) for 3.5 hours.

MEASUREMENTS AND RESULTS

The dim light melatonin onset was assessed before and after the 3-day treatment. Subjects completed daily sleep logs and symptom questionnaires and wore wrist activity monitors. The dim light melatonin onset advanced more in the 2-hours-per-day group than in the 1-hour-per-day group (median phase advances of 1.9 and 1.4 hours), but the difference between the means (1.8 and 1.5 hours) was not statistically significant. By the third treatment day, circadian rhythms were misaligned relative to the sleep schedule, and subjects had difficulty falling asleep in the 2-hours-per-day group, but this was not the case in the 1-hour-per-day group. Nevertheless, the 2-hours-per-day group did slightly better on the symptom questionnaires. In general, sleep disturbance and other side effects were small.

CONCLUSIONS

A gradually advancing sleep schedule with intermittent morning bright light can be used to advance circadian rhythms before eastward flight and, thus, theoretically, prevent or reduce subsequent jet lag. Given the morning light treatment used here, advancing the sleep schedule 2 hours per day is not better than advancing it 1 hour per day because it was too fast for the advance in circadian rhythms. A diagram is provided to help the traveler plan a preflight schedule.

Comparison of amplitude recovery dynamics of two limit cycle oscillator models of the human circadian pacemaker

At an organism level, the mammalian circadian pacemaker is a two-dimensional system. For these two dimensions, phase (relative timing) and amplitude of the circadian pacemaker are commonly used. Both the phase and the amplitude (A) of the human circadian pacemaker can be observed within multiple physiological measures--including plasma cortisol, plasma melatonin, and core body temperature (CBT)--all of which are also used as markers of the circadian system. Although most previous work has concentrated on changes in phase of the circadian system, critically timed light exposure can significantly reduce the amplitude of the pacemaker. The rate at which the amplitude recovers to its equilibrium level after reduction can have physiological significance. Two mathematical models that describe the phase and amplitude dynamics of the pacemaker have been reported. These models are essentially equivalent in predictions of phase and in predictions of amplitude recovery for small changes from an equilibrium value (A = 1), but are markedly different in the prediction of recovery rates when A < 0.6. To determine which dynamic model best describes the amplitude recovery observed in experimental data; both models were fit to CBT data using a maximum likelihood procedure and compared using Akaike's Information Criterion (AIC). For all subjects, the model with the lower recovery rate provided a better fit to data in terms of AIC, supporting evidence that the amplitude recovery of the endogenous pacemaker is slow at low amplitudes. Experiments derived from model predictions are proposed to test the influence of low amplitude recovery on the physiological and neurobehavioral functions.

Efficacy of a single sequence of intermittent bright light pulses for delaying circadian phase in humans

It has been shown in animal studies that exposure to brief pulses of bright light can phase shift the circadian pacemaker and that the resetting action of light is most efficient during the first minutes of light exposure. In humans, multiple consecutive days of exposure to brief bright light pulses have been shown to phase shift the circadian pacemaker. The aim of the present study was to determine whether a single sequence of brief bright light pulses administered during the early biological night would phase delay the human circadian pacemaker. Twenty-one healthy young subjects underwent a 6.5-h light exposure session in one of three randomly assigned conditions: 1) continuous bright light of approximately 9,500 lux, 2) intermittent bright light (six 15-min bright light pulses of approximately 9,500 lux separated by 60 min of very dim light of <1 lux), and 3) continuous very dim light of <1 lux. Twenty subjects were included in the analysis. Core body temperature (CBT) and melatonin were used as phase markers of the circadian pacemaker. Phase delays of CBT and melatonin rhythms in response to intermittent bright light pulses were comparable to those measured after continuous bright light exposure, even though the total exposure to the intermittent bright light represented only 23% of the 6.5-h continuous exposure. These results demonstrate that a single sequence of intermittent bright light pulses can phase delay the human circadian pacemaker and show that intermittent pulses have a greater resetting efficacy on a per minute basis than does continuous exposure.

Suppression of sleepiness and melatonin by bright light exposure during breaks in night work

Night work is non-optimal for performance and recuperation because of a lack of circadian influence that fully promote a night orientation. Our study assessed, in an industrial setting, the effects of bright light exposure (BL) on sleepiness, sleep and melatonin, during night work and during the following readaptation to day work. In a crossover design, 18 workers at a truck production plant were exposed to either BL (2500 lx) during breaks or normal light during four consecutive weeks. Twenty minute breaks were initiated by 67% of the workers between 03:00 and 04:00 hours. Sleep/wake patterns were assessed through actigraphs and ratings were given in a sleep/wake diary. Saliva melatonin was measured at 2-h intervals before, during and after night shift weeks. A significant interaction demonstrated a reduction of sleepiness in the BL condition particularly on the first two nights at 04:00 and 06:00 hours. Day sleep in the BL condition was significantly lengthened. Bright light administration significantly suppressed melatonin levels during night work and most strongly at 02:00 hours. Daytime melatonin during the readaptation after night work remained unaffected. The present findings demonstrate the feasibility and benefits of photic stimulation in industrial settings to increase adaptation to night work.

Preflight adjustment to eastward travel: 3 days of advancing sleep with and without morning bright light

Jet lag is caused by a misalignment between circadian rhythms and local destination time. As humans typically take longer to re-entrain after a phase advance than a phase delay, eastward travel is often more difficult than westward travel. Previous strategies to reduce jet lag have focused on shaping the perceived light-dark cycle after arrival, in order to facilitate a phase shift in the appropriate direction. Here we tested treatments that travelers could use to phase advance their circadian rhythms prior to eastward flight. Thus, travelers would arrive with their circadian rhythms already partially re-entrained to local time. We determined how far the circadian rhythms phase advanced, and the associated side effects related to sleep and mood. Twenty-eight healthy young subjects participated in 1 of 3 different treatments, which all phase advanced each subject's habitual sleep schedule by 1 h/day for 3 days. The 3 treatments differed in morning light exposure for the 1st 3.5 h after waking on each of the 3 days: continuous bright light (> 3000 lux), intermittent bright light (> 3000 lux, 0.5 h on, 0.5 off, etc.), or ordinary dim indoor light (< 60 lux). A phase assessment in dim light (< 10 lux) was conducted before and after the treatments to determine the endogenous salivary dim light melatonin onset (DLMO). The mean DLMO phase advances in the dim, intermittent, and continuous light groups were 0.6, 1.5, and 2.1 h, respectively. The intermittent and continuous light groups advanced significantly more than the dim light group (p < 0.01) but were not significantly different from each other. The side effects as assessed with actigraphy and logs were small. A 2-h phase advance may seem small compared to a 6- to 9-h time zone change, as occurs with eastward travel from the USA to Europe. However, a small phase advance will not only reduce the degree of re-entrainment required after arrival, but may also increase postflight exposure to phase-advancing light relative to phase-delaying light, thereby reducing the risk of antidromic re-entrainment. More days of preflight treatment could be used to produce even larger phase advances and potentially eliminate jet lag.

Integration of human sleep-wake regulation and circadian rhythmicity

The human sleep-wake cycle is generated by a circadian process, originating from the suprachiasmatic nuclei, in interaction with a separate oscillatory process: the sleep homeostat. The sleep-wake cycle is normally timed to occur at a specific phase relative to the external cycle of light-dark exposure. It is also timed at a specific phase relative to internal circadian rhythms, such as the pineal melatonin rhythm, the circadian sleep-wake propensity rhythm, and the rhythm of responsiveness of the circadian pacemaker to light. Variations in these internal and external phase relationships, such as those that occur in blindness, aging, morning and evening, and advanced and delayed sleep-phase syndrome, lead to sleep disruptions and complaints. Changes in ocular circadian photoreception, interindividual variation in the near-24-h intrinsic period of the circadian pacemaker, and sleep homeostasis can contribute to variations in external and internal phase. Recent findings on the physiological and molecular-genetic correlates of circadian sleep disorders suggest that the timing of the sleep-wake cycle and circadian rhythms is closely integrated but is, in part, regulated differentially.

A multifactorial approach employing melatonin to accelerate resynchronization of sleep-wake cycle after a 12 time-zone westerly transmeridian flight in elite soccer athletes

Rapid transmeridian translocation through multiple time zones has a negative impact on athletic performance. The aim of the present study was to test the timely use of three factors (melatonin treatment, exposure to light, physical exercise) to hasten the resynchronization of a group of elite sports competitors and their coaches to a westerly transmeridian flight comprising of 12 time-zones. Twenty-two male subjects were included in the study. They were professional soccer players and their coaches who travelled to Tokyo to play the final game of the Intercontinental Coup. The day prior to departure, urine was collected from each subject from 18:00 to 06:00 hrs to measure the melatonin metabolite 6-sulphatoxymelatonin. Participants were asked to complete sleep log diaries from day 0 (preflight) to the day before returning to Buenos Aires (day 8). All subjects received 3 mg of melatonin p.o. daily at expected bedtime at Tokyo immediately after leaving Buenos Aires. Upon arrival at Tokyo the subjects performed a daily physical exercise routine outdoors at two restricted times of the day (from 08:00 to 11:00 hrs in the morning and from 13:00 to 16:00 hrs in the afternoon). Exposure to sunlight or physical exercise at other times of the day was avoided. Except for the number of awakenings (which increased on days 1 and 3) and sleep latency (which decreased on days 2, 6 and 8), there was an absence of significant changes in subjective sleep parameters as compared with preflight assessment. Sleep quality and morning alertness at Tokyo correlated significantly with preflight 6-sulphatoxymelatonin excretion. Mean resynchronization rate of sleep-wake cycle to the 12 hr-time shift was 2.13 +/- 0.88 days, significantly different from the minimal resynchronization rate of 6 days expected after a 12-time-zones flight. The results indicate that the combination of melatonin treatment, an appropriate environmental light schedule and timely applied physical exercise can be useful to help elite athletes to overcome the consequences of jet lag.

Phase-advance shifts of human circadian pacemaker are accelerated by daytime physical exercise

Effects of forced sleep-wake schedules with and without physical exercise were examined on the human circadian pacemaker under dim light conditions. Subjects spent 15 days in an isolation facility separately without knowing the time of day and followed a forced sleep-wake schedule of a 23 h 40-min period for 12 cycles, and physical exercise was imposed twice per waking period for 2 h each with bicycle- or rowing-type ergometers. As a result, plasma melatonin rhythm was significantly phase advanced with physical exercise, whereas it was not changed without exercise. The difference in phase was already significant 6 days after the start of exercise. The amplitude of melatonin rhythm was not affected. A single pulse of physical exercise in the afternoon or at midnight significantly phase delayed the melatonin rhythms when compared with the prepulse phase, but the amount of phase shift was not different from that observed in the sedentary controls. These findings indicate that physical exercise accelerates phase-advance shifts of the human circadian pacemaker associated with the forced sleep-wake schedule.