The endogenous circadian rhythm has a cycle of around 24.2 h, but its fine tuning to the environment is achieved through the influence of external time givers of which the most important is light (Fig. 5.1). The fluctuations in the circadian sleep rhythm alter the threshold of the homeostatic and adaptive drives to initiate sleep. Disturbances in circadian sleep rhythms can therefore cause either insomnia or excessive daytime sleepiness, or both.
The sleep, temperature and hormonal circadian rhythms are usually synchronized and are synergistic in promoting sleep and wakefulness at different points in each circadian cycle. This synchrony can break down in certain situations because each rhythm adapts to changes in external factors at different rates and to different degrees. The circadian sleep rhythm could, for instance, be promoting sleep at a time when the temperature is rising instead of falling as is usual late in the evening, and when catabolic rather than anabolic hormones are being secreted. The conflicting effects of this 'internal desynchronization' disturb sleep and reduce the level of alertness during wakefulness.
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