Circadian rhythms adaptation

Martin SK, Eastman CI. Medium-intensity hght produces circadian rhythm adaptation to simulated night-shift work. Sleep 1998 21 154—165. 

Recent experiments (4) have shown that there are no significant age or gender-related differences in thermal environment preference when all other factors such as weight of clothing and activity level are the same. Whereas people often accept thermal environments outside of their comfort range, there is no evidence that they adapt to these other conditions. Their environmental preference does not change. Similarly there is no evidence that there is any seasonal or circadian rhythm influence on a person s thermal preference. 

The most ubiquitous biological rhythms are those that occur with a period close to 24 h in all eukaryotes and in some prokaryotes such as cyanobacteria. These circadian rhythms allow organisms to adapt to the natural periodicity of the terrestrial environment, which is characterized by the alternation of day and night due to rotation of the earth on its axis. Circadian clocks provide cells with an endogenous mechanism, allowing them to anticipate the time of day. 

FIGURE 29-3 Circadian rhythm of cortisol production in humans. Peak plasma cortisol levels normally occur approximately at the time an individual awakens (6 to 8 am). (Adapted from Katzung BG. Basic and Clinical Pharmacology. 2nd ed. Lange Medical Publications New York 1984 454 after Liddle, 1966. Reproduced with permission of the McGraw-Hill Companies.)... 

We should not assume that odor coding at the level of the sensory neurons is a static phenomenon. Research on Drosophila gives us three cases where influences from the external or internal environment change response properties of ORNs. First, sensory adaptation can temporarily reset sensitivity of ORNs to persisting odorants. Next, circadian rhythms can cause daily cycles of ORN sensitivity. Finally, sensory neurons may change their synaptic properties as a result of chronic odor exposure. 

While adaptation acts over minutes, circadian rhythms act over hours. The EAG amplitude to ethyl acetate and benzaldehyde varies with circadian rhythm... 

Fig. 13.1. Schematic diagram of the behavioral sleep-wake rhythm. This cycle (termed circadian rhythm) represents the fundamental adaptation of organisms to an environmental stimulus, the daily cycle of light and dark. Here the circadian rhythm is shown entrained for five days by the environmental light-dark cycle and autonomous for the rest of the experiment when the subject is placed under constant light conditions. The intrinsic period of the circadian oscillator is in this particular case greater than 24 hours. Correspondingly, the phase difference between the sleep-wake cycle and daily cycle increases the internal day begins later and latei-. Such plots are typically oljserved in experiments with both animals and humans, see, c.g., [4, 15, 33. ...

As indicated in fig. 2.29, subharmonic entrainment at a fraction of the fundamental frequency, which is characteristic of nonlinear systems (Hayashi, 1964), occurs in smaller and smaller domains of the external period as the value of that fraction diminishes. A similar phenomenon is observed in the entrainment of circadian rhythms by light-dark cycles (Pittendrigh, 1960,1965) the latter property imderlies the adaptation of most living organisms to periodic variations in their environment. 

The most important characteristic of the human circadian system for the shiftworker is that much of it is resistant to abrupt changes in schedule. This means that the circadian rhythm will continue to cycle in a way that is not adapted to current working patterns for some time, possibly weeks or even months. Note that circadian rhythms never fully adapt, even in cases where the operator is working on a permanent night shift The practical effect of this is that the operator s circadian rhythm can be caUing for sleep when... 

Miki K, Sudo A (1996) Adaptation of circadian corticosterone and catecholamine rhythms to light-dark cycle reversal in the rat. Ind-Health 34 133-138... 

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