Differential responses to artificial photoperiods of the rising and falling phases of human melatonin rhythm are consistent with a dual oscillator hypothesis

From the Pineal Gland to the Central Clock in the Brain: Beginning of Studies of the Mammalian Biological Rhythms in the Institute of Physiology of the Czech Academy of Sciences

The Institute of Physiology of the Czech Academy of Sciences (CAS) has been involved in the field of chronobiology, i.e., in research on temporal regulation of physiological processes, since 1970. The review describes the first 35 years of the research mostly on the effect of light and daylength, i.e., photoperiod, on entrainment or resetting of the pineal rhythm in melatonin production and of intrinsic rhythms in the central biological clock. This clock controls pineal and other circadian rhythms and is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. During the early chronobiological research, many original findings have been reported, e.g. on mechanisms of resetting of the pineal rhythm in melatonin production by short light pulses or by long exposures of animals to light at night, on modulation of the nocturnal melatonin production by the photoperiod or on the presence of high affinity melatonin binding sites in the SCN. The first evidence was given that the photoperiod modulates functional properties of the SCN and hence the SCN not only controls the daily programme of the organism but it may serve also as a calendar measuring the time of a year. During all the years, the chronobiological community has started to talk about "the Czech school of chronobiology". At present, the today´s Laboratory of Biological Rhythms of the Institute of Physiology CAS continues in the chronobiological research and the studies have been extended to the entire circadian timekeeping system in mammals with focus on its ontogenesis, entrainment mechanisms and circadian regulation of physiological functions. Key words: Pineal, Melatonin, AA-NAT rhythm, Light entrainment, Photoperiod, SCN clock.

Two oscillatory components detected by forced splitting of the sleep-wake cycle in humans

The sleep-wake cycle of human subjects was artificially split into two episodes by imposing an 8-h light and 4-h dark cycle (LD 8:4) twice a day for 7 days, which was followed by a 3-day free-running session. Sleep was permitted only in the dark period. The subjects in the ordinary group were exposed to ordinary light (ca. 500 lx) in the 8-h light period, and those in the bright light group to bright (ca. 5,000 lx) and ordinary light alternatively with bright light after the first dark period (2400-400). Split sleeps persisted in the free-running session with the major episode around the first dark period and the minor episode around the second dark period. By contrast, circadian melatonin rhythm in the free-running session significantly phase delayed in the ordinary light group, but phase advanced in the bright light group, keeping the melatonin rhythm unsplit. The length of nocturnal melatonin secretion (NMS) was significantly shortened in the bright light group. Interestingly, the falling phase of NMS advanced significantly further than the rising phase. Such a difference was not detected in the ordinary light group. Similar differences were observed in the body temperature rhythm. These findings indicated oscillatory mechanisms underlying split sleeps distinct from the circadian pacemaker and suggested an involvement of different circadian oscillators in the rising and falling phases of NMS, which is consistent with the dual oscillator model proposed for the circadian system of nocturnal rodents.NEW & NOTEWORTHY The present study demonstrated that human sleep was separated into two essentially identical components, which persisted under constant conditions, suggesting circadian oscillator underlying split-sleep episodes. The study also indicated differential light sensitivities in the rising and falling phases of circadian melatonin rhythm, indicating the involvement of two different oscillators. These results consisted of the evening and morning dual-oscillator hypothesis for the circadian pacemaker and the hierarchical model for the pacemaker and sleep-wake cycle.