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obrietan3 - BASIC SCIENCE FOR THE CLINICIAN Recent Insights...

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INTRODUCTION SLEEP AND CIRCADIAN RHYTHMICITY ARE INEXTRICABLY INTERTWINED; THE CIRCADIAN CLOCK EXERTS A POWER- FUL INFLUENCE ON THE TIMING OF SLEEP AND WAKEFUL- NESS, WHILE MORE RECENT LITERATURE SUGGESTS THAT SLEEP STATES CAN INFLUENCE THE FUNCTION OF THE CIR- CADIAN PACEMAKER. It thus behooves the sleep disorders physician or researcher to maintain familiarity with work from the circadian-clock research community. Unfortunately, the explosion of literature in this field in the past several years (numbering more than 7000 publications listed on PubMed since the year 2000 alone) has made staying current with recent developments a daunting task even for the cognoscenti in the field. This review will attempt to highlight some of the recent research in the field of mammalian biologic rhythms; it is not meant to be a com- prehensive review of the field. The interested reader is referred to an excellent recent textbook in the field. 1 The interaction of human circa- dian rhythms and sleep will be covered in a subsequent review. BASIC DEFINITIONS Circadian rhythms are defined as the self-sustained oscillations of liv- ing systems that display near–24-hour periodicity when the system is kept away from all external time cues (see Pittendrigh 2 for an excellent overview). Circadian rhythms typically have 4 cardinal properties: z They persist (by definition) in the absence of external time cues, demonstrating a characteristic free-running period. z The phase of the rhythm can be shifted by application of light or drugs. For light, nearly all organisms have a similar phase response to short stimuli, being insensitive to light presented during the sub- jective day (that is, the times when the animal would normally be exposed to daylight), showing a delay in the phase with light in the early part of the subjective night, and showing an advance in the phase of the rhythm in the late subjective night. z The period (and phase) of the rhythm can be entrained by periodic stimuli, such as periodic light-dark cycles, provided their period is near the intrinsic free-running period of the clock. z The clock is temperature compensated, meaning that its free-run- ning rhythm does not vary markedly with changes in the ambient temperature. This applies primarily to poikilothermic species, not to mammals. Circadian rhythms have been found in organisms ranging from single- celled photosynthetic prokaryotes 3 through humans, and in nearly every eukaryotic organism in which they have been sought. In mammals, they influence a large number of physiologic, endocrinologic, and biochemi- cal processes, including of course the timing of sleep and wakefulness. ANATOMIC FEATURES OF THE MAMMALIAN CIRCADIAN CLOCK While the formal properties of the circadian clock (that is, the nature of the free-running period, the phase response of the clock to light, etc.) had been well described in many species by the late 1960s, the mecha- nism of circadian timekeeping remained quite mysterious. The first
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