Periodic oscillations (rhythms) have been documented in biological variables in a whole spectrum of living organisms (from unicellular to multicellular).1, 2 However, this phenomenon is not merely a reaction to environmental changes; it is generally held that the rhythms are governed by an active system capable of self-sustained oscillations (TGX-221 purchase endogenous rhythms).1 Consequently,

the shape of rhythms and the temporal order are products of the interaction between endogenous (genetically controlled) oscillators and the phases (synchronizing, entraining) Inhibitors,research,lifescience,medical of external cues. Features of biological rhythm The parameters of a biological rhythm are as follows1-6 : The period τ (τ=24 h in circadian rhythm; and τ<20 h in ultradian rhythm). The acrophase (Φ, the peak time of the rhythm). This parameter usually includes a phase reference within the time axis of the rhythm (eg, for the circadian rhythm the acrophase relates to a phase reference like midnight, local time, or mid-sleep). The amplitude Inhibitors,research,lifescience,medical (A), the pcak-to-trough difference. The mean level, or mesor (M). Rhythms that follow a cosine curve can be characterized by all four of these parameters, and rhythms that do not follow cosine shape are mostly characterized

by M and τ. The majority of the rhythms studied in nature, and especially in humans, exhibit circadian periodicity, Inhibitors,research,lifescience,medical and this review will focus mainly on these (though most of discussions herein also apply to rhythms

with other periodicities). Circadian rhythms have the following properties1-8 : They have a Inhibitors,research,lifescience,medical genetic origin. They are controlled by biological clocks (or oscillators or circadian pacemakers). The biological clocks are reset (Φ) and calibrated (τ=24 h) by environmental signals that also have τ=24 h, such Inhibitors,research,lifescience,medical as dawn/dusk (photic signals), activity /rest, or noi.se/silcncc (nonphotic signals). These periodic environmental factors are called synchronizers,9 zeitgebers,10 or entraining agents.7 The range of period cntrainment of circadian rhythms by the zeitgebers may vary between τ=20 h and τ=28 h. There is a general ubiquity 7, 8 of the properties of the biological rhythms quoted above, from unicellular eukaryotes8-11-12 Astemizole to humans.2, 5, 13 However, some variability exists and some differences can be observed among plants,12 animals,13 strains of the same species,14 and even different human individuals.5, 13, 15, 16 The master clock versus temporal organization In recent years, a large amount of information has accumulated about the genetic, molecular, physiological, and environmental induction of biological rhythms and about how they function in various genera and species. Due to the variety and variability of this vast literature, it is no longer an easy task to review concepts in human biological rhythms. We will first try to present the reasons for this difficulty. Two schools of thoughts coexist in chronobiology.