The diurnal cycling of sleep–wake rhythms maintains sleep and wakefulness in line with environmental time, but by itself is insufficient to initiate sleep (

Fig. 2.8).

Loss of function of the SCN increases the duration of sleep, indicating that it has a wakefulness maintaining effect and tends to counteract the homeostatic drive to sleep. In effect it raises the threshold of the other sleep drives for initiating sleep. It does not selectively promote either NREM or REM sleep.

The SCN promotes wakefulness during the day, except between 2.00 and 4.00 pm, and particularly in the evenings shortly before the usual time of sleep onset which is when the homeostatic drive to sleep is greatest. The circadian rhythm opposes this and leads to the ‘wakefulness maintenance zone’ or ‘forbidden zone’.

At this time just prior to the usual time of sleep it is difficult to fall asleep. The end point of this zone occurs when the homeostatic drive, together with an input from the adaptive drive, overcomes this circadian influence. The onset of secretion of melatonin by the pineal gland also inhibits the SCN activity.

The circadian drive to wakefulness is at its least before the habitual wake-up time of the subject in the morning. This helps to consolidate sleep at the end of the night when the homeostatic drive to sleep is waning. At the time of waking the circadian drive to wakefulness increases sharply and its wakefulness effect overcomes any residual homeostatic drive to sleep.