Circadian regulation of human sleep and age-related changes in its timing,consolidation and EEG characteristics

The light-entrainable circadian pacemaker located in the suprachiasmatic nu cleus of the hypothalamus regulates the timing and consolidation of sleep b y generating a paradoxical rhythm of sleep propensity; the circadian drive for wakefulness peaks at the end of the day spent awake, ie close to the on set of melatonin secretion at 21.00-22.00 h and the circadian drive for sle ep crests shortly before habitual waking-up time. With advancing age, ie af ter early adulthood, sleep consolidation declines, and time of awakening an d the rhythms of body temperature, plasma melatonin and cortisol shift to a n earlier clock hour. The variability of the phase relationship between the sleep-wake cycle and circadian rhythms increases, and in old age sleep is more susceptible to internal arousing stimuli associated with circadian mis alignment. The propensity to awaken from sleep advances relative to the bod y temperature nadir in older people, a change that is opposite to the phase delay of awakening relative to internal circadian rhythms associated with morningness in young people. Age-related changes do not appear to be associ ated with a shortening of the circadian period or a reduction of the circad ian drive for wake maintenance. These changes may be related to changes in the sleep process itself, such as reductions in slow-wave sleep and sleep s pindles as well as a reduced strength of the circadian signal promoting sle ep in the early morning hours. Putative mediators and modulators of circadi an sleep regulation are discussed.