Temperature cycle
The sleep and temperature cycles are usually closely coupled _[18], although in free-running experiments the sleep cycle usually becomes around 1h longer than the temperature cycle. The temperature of the blood is sensed in the paraventricular nucleus which is close to, but separate from, the VLPO and other sleep-regulating sensors in the preoptic area of the anterior hypothalamus. Activity in these centres suppresses arousals and increases NREM sleep as well as reducing the core body temperature. Melatonin secretion facilitates sleep and also lowers the core body temperature. The peak melatonin level is seen at the nadir of the core body temperature at 3.00–5.00 am.

The paraventricular nucleus reacts to a rise in temperature by increasing heat loss through cutaneous vasodilatation and sweating. If the temperature falls, the posterior hypothalamus causes heat conservation by cutaneous vasoconstriction and increased heat production through skeletal muscle activity, including shivering, and piloerection.

The amplitude of the circadian temperature cycle is 0.5–0.75°C, although this is reduced in the elderly.

The temperature rises to a peak of 37°C at 3.00–5.00 pm and falls only slowly until around 9.00 pm. The drop in core temperature which precedes sleep is associated with an elevation of skin temperature due to cutaneous vasodilatation, possibly related to accumulation of adenosine in the basal forebrain.

The temperature continues to fall after sleep onset due to a reduction in physical activity and metabolic rate and to vasodilatation during NREM sleep. The assumption of the supine position also lowers the temperature through a postural reflex. The temperature nadir is usually at 3.00–5.00 am, which coincides with the phase change in response to light (

Fig. 2.9).

The subsequent temperature rise is linked to the increase in duration and density of REM sleep, although thermoregulation is much less precisely controlled in REM sleep than in NREM sleep or wakefulness.