13, 30, 31 Recent studies show that even human rhythms can be ent

13, 30, 31 Recent studies show that even human rhythms can be entrained by low intensity light.32 Another example that illustrates the confusion in defining a concept due to a focus on the rhythm of one variable rather than on the temporal order is the following. In the 1970s, most sleep studies were extensively

carried on cats, using electroencephalography (EEG). It was shown Inhibitors,research,lifescience,medical that most individuals of this species are frequent sleepers, with a polyphasic rhythmicity. According to Jouvet,33 no more than 30% of cats exhibit a sleep/wake rhythm with τ=24 h. As a result, it was believed by some authors that cat is a species that does not possess a circadian organization – an idea that was a source of conflict between sleep and biological rhythm specialists. However, cats exhibit circadian rhythms in their feeding behavior and activity/rest rhythm.34, 35 It proved difficult to bridge the gap between those involved in sleep research in cats and those studying circadian rhythms in laboratory Inhibitors,research,lifescience,medical rodents.33 The final source of misunderstanding in concept definition relates to the fact that the meaning of a given term evolves

as time passes. Let us take the term chronobiolic as an example.25, 26, 36-38 Simpson et al36 hypothesized that a drug might be able to phase shift all circadian rhythms by resetting their respective Os. In fact, there is still Inhibitors,research,lifescience,medical no such wonderdrug.37, 38 Thereafter, the meaning of the term check details chronobiotic was restricted to a drug able to phase shift or reset one39 or a limited number25-26 of rhythms. The latter demonstrates once again the importance of studying systems or temporal order rather than just one rhythm. Considering the above examples, Inhibitors,research,lifescience,medical the definitions and

concepts presented in this paper have been updated with reference to the recent state of art. Temporal organization Temporal organization refers to the sequential array of rhythms of various variables, each with a specific phase on the time axis. An Inhibitors,research,lifescience,medical examination of the array provides information about the phase relationship between the rhythms, but does not show whether there is a causal Electron transport chain interaction between them. One example is body temperature rhythm and paradoxical sleep (PS) or rapid eye movement (REM) rhythm. In humans, a physiological trough in temperature coincides in time with the longest episodes of PS.40, 41 Animal experiments have demonstrated that hypothermia influences PS.42 Thus, while a phase relationship between the two rhythms docs not in itself imply a causal relationship, the physiological interaction between the two variables raises the strong possibility that they are coupled to the same oscillator or that there is a causal interaction between the two. A simplified circadian map of the temporal cardiovascular organization of healthy young adults provides another example (Figure 1).

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