Nonphotic entrainment in humans?

Although light is accepted as the dominant zeitgeber for entrainment of the human circadian system, there is evidence that nonphotic stimuli may play a role. This review critically assesses the current evidence in support of nonphotic entrainment in humans. Studies involving manipulations of sleep-wake schedules, exercise, mealtimes, and social stimuli are re-examined, bearing in mind the fact that the human circadian clock is sensitive to very dim light and has a free-running period very close to 24 h. Because of light confounds, the study of totally blind subjects with free-running circadian rhythms represents the ideal model to investigate the effects of nonphotic stimuli on circadian phase and period. Strong support for nonphotic entrainment in humans has already come from the study of a few blind subjects with entrained circadian rhythms. However, in these studies the nonphotic stimulus(i) responsible was not identified. The effect of appropriately timed exercise or exogenous melatonin represents the best proof to date of an effect of nonphotic stimuli on human circadian timing. Phase-response curves for both exercise and melatonin have been constructed. Given the powerful effect of feeding as a circadian zeitgeber in various nonhuman species, studies of meal timing are recommended. In conclusion, the available evidence indicates that it remains worthwhile to continue to study nonphotic effects on human circadian timing to identify treatment strategies for shift workers and transmeridian travelers as well as for the blind and possibly the elderly.     

Sleep propensity is modulated by circadian and behavior-induced changes in cutaneous temperature

A close relation between sleep and body temperature has been noted already for a long time. Although a correlation is indisputable, there is at present hardly evidence for a causal involvement of sleep in changes in body temperature. Concerning the reverse, a causal involvement of body temperature in sleep has been demonstrated: if core or skin temperature changes activate thermoregulatory processes aimed at heat loss or heat preservation, sleep is usually disrupted. We have recently proposed that sleep propensity is also affected by more subtle changes in skin temperature, within the thermoneutral range (Van Someren (2000). Chronobiol. Int. 17, 313–354). These changes are likely to modulate the firing properties of thermosensitive neurons in brain areas involved in sleep regulation. Subtle changes in skin temperature occur daily under control of the circadian timing system. They could provide this system with an additional signal pathway to support its neuronal and neurohormonal signals to enforce circadian modulation of sleep propensity. Subtle changes in skin temperature also result from behavior, and could contribute to the changes in sleep propensity resulting from these behaviors. The present review summarizes the neurobiological background and correlational physiological and behavioral data in support of the involvement of skin temperature in the modulation of sleep propensity. It moreover points out the type of experimental investigations needed to support or refute the hypothesis.     

Acute exposure to 2G phase shifts the rat circadian timing system

The circadian timing system (CTS) provides internal and external temporal coordination of an animal's physiology and behavior. In mammals, the generation and coordination of these circadian rhythms is controlled by a neural pacemaker, the suprachiasmatic nucleus (SCN), located within the hypothalamus. The pacemaker is synchronized to the 24 hour day by time cues (zeitgebers) such as the light/dark cycle. When an animal is exposed to an environment without time cues, the circadian rhythms maintain internal temporal coordination but exhibit a "free-running" condition in which the period length is determined by the internal pacemaker. Maintenance of internal and external temporal coordination are critical for normal physiological and psychological function in human and non-human primates. Exposure to altered gravitational environments has been shown to affect the amplitude, mean, and timing of circadian rhythms in species ranging from unicellular organisms to man. However, it has not been determined whether altered gravitational fields have a direct effect on the neural pacemaker, or affect peripheral physiological systems that express these circadian parameters. In previous studies, the ability of a stimulus to phase shift circadian rhythms was used to determine whether a stimulus has a direct effect on the neural pacemaker. The present experiment was performed in order to determine whether acute exposure to a hyperdynamic field could phase shift circadian rhythms.     

斑马鱼生物钟研究进展

斑马鱼是生物钟研究领域中一种新兴的脊椎动物模型。文章总结了斑马鱼生物钟研究的一些进展, 以及利用斑马鱼研究生物钟的特点及优势。由于光照和温度作为重要的外部信号在斑马鱼生物钟调节中发挥重要作用, 文章主要就近期光和温度对斑马鱼钟基因及调节通路的研究进行了概述, 最后对斑马鱼生物钟研究的未来提出了展望。     

Insect quality control: synchronized sex,mating system,and biological rhythm

The sterile insect technique (SIT) is a method of eradicating insects by releasing mass-reared sterilized males into fields to reduce the hatchability of eggs laid by wild females that have mated with the sterile males. SIT requires mass-production of the target insect, and maintenance of the quality of the mass-reared insects. The most important factor is successful mating between wild females and sterile males because SIT depends on their synchronized copulation. Therefore, understanding the mating systems and fertilization processes of target insects is prerequisite. Insect behavior often has circadian rhythms that are controlled by a biological clock. However, very few studies of relationships between sterile insect quality and circadian rhythm have been performed compared with the amount of research on the mating ability of target insects. The timing of male copulation attempts with receptivity of females is key to successful mating between released males and wild females. Therefore, we should focus on the mechanisms controlling the timing of mating in target insects. On the other hand, in biological control projects, precise timing of the release of natural enemies to attack pest species is required because behavior of pests and control agents are affected by their circadian rhythms. Involving both chronobiologists and applied entomologists might produce novel ideas for sterile insect quality control by synchronized sex between mass-reared and wild flies, and for biological control agent quality by matching timing in activity between predator activity and prey behavior. Control of the biological clocks in sterile insects or biological control agents is required for advanced quality control of rearing insects.     

Evolution of time-keeping mechanisms: early emergence and adaptation to photoperiod

Virtually all species have developed cellular oscillations and mechanisms that synchronize these cellular oscillations to environmental cycles. Such environmental cycles in biotic (e.g. food availability and predation risk) or abiotic (e.g. temperature and light) factors may occur on a daily, annual or tidal time scale. Internal timing mechanisms may facilitate behavioural or physiological adaptation to such changes in environmental conditions. These timing mechanisms commonly involve an internal molecular oscillator (a 'clock') that is synchronized ('entrained') to the environmental cycle by receptor mechanisms responding to relevant environmental signals ('Zeitgeber', i.e. German for time-giver). To understand the evolution of such timing mechanisms, we have to understand the mechanisms leading to selective advantage. Although major advances have been made in our understanding of the physiological and molecular mechanisms driving internal cycles (proximate questions), studies identifying mechanisms of natural selection on clock systems (ultimate questions) are rather limited. Here, we discuss the selective advantage of a circadian system and how its adaptation to day length variation may have a functional role in optimizing seasonal timing. We discuss various cases where selective advantages of circadian timing mechanisms have been shown and cases where temporarily loss of circadian timing may cause selective advantage. We suggest an explanation for why a circadian timing system has emerged in primitive life forms like cyanobacteria and we evaluate a possible molecular mechanism that enabled these bacteria to adapt to seasonal variation in day length. We further discuss how the role of the circadian system in photoperiodic time measurement may explain differential selection pressures on circadian period when species are exposed to changing climatic conditions (e.g. global warming) or when they expand their geographical range to different latitudes or altitudes.     

Sharing time on the fly

The investigation of circadian clock function in Drosophila has progressed from the identification of clock genes to the analysis of timing mechanisms in the cells and tissues where these genes are expressed. As the biological context for investigating circadian clock systems is expanded, new features of molecular timing mechanisms are becoming apparent. Examples come first from studies on peripheral clocks, which perform local, tissue-specific functions as well as global functions that relate to the control of individual behavior, and second from the evaluation of social influences on circadian rhythms. The identification of inter-organismal components of the circadian system in Drosophila suggests new perspectives as the progression continues from the systems level to the social level and onwards to the level of ecosystems.