The pacemaker neurons responsible for this internal clock are found in a small area on the underside of the brain, known as the suprachiasmatic nucleus, which receives direct input from the eyes, as well as from many regions of the brain. Although the length of the daily clock cycle does not change in older people, their physiological rhythms, including the time of waking, tend to shift to earlier in the day. At the other end of life, sleep in the newborn may have a special function in brain development. The EEG in rat pups and human babies shows no periods of quiet sleep characterised by slow waves, only activity generally considered to be that of REM sleep. Careful analysis of this 'active sleep' in very young rat pups has now revealed that it differs from adult REM sleep.
Complex mechanisms exist to maintain the balance between waking activity, quiet sleep and REM sleep.
After sleep deprivation, the balance is restored by increasing the amount and depth of quiet sleep. A considerable surprise is that inactivity also creates a need for sleep. Animals that conserve their energy in winter by hibernating have low body temperature and very reduced brain activity. Periodically their temperature returns to normal, when they spend most of their time in quiet sleep, as if they were sleep deprived. The Djungarian hamster, a native of Siberia, does not hibernate but has daily periods of torpor, during which body temperature and brain activity are both reduced.