The activity of the cerebral cortex is critical in determining whether sleep or wakefulness occurs, but it does not generate the drive to enter sleep or wakefulness.

This is due to the…

The pineal gland contains a circadian pacemaker in some vertebrates, such as perching birds (passerines).

In humans this function is taken over by the SCN in the anterior hypothalamus, which controls the…

Fibres leave the SCN to reach the ventrolateral preoptic area (VLPO) of the anterior hypothalamus multisynaptically. They initially relay in the sub-paraventricular zone before synapsing predominantly in the dorsomedial nucleus of…

The clock in the SCN, through its neurological connections, has a circadian influence not only on sleep, but also on temperature control, hormone secretion and other functions. The multisynaptic processing of the SCN…

Circadian rhythm periodicity
Circadian rhythms have a periodicity of around a day, which is longer than ultradian rhythms of,  for instance, REM sleep and shorter than infradian rhythms such as the menstrual cycle. The…

Impulses reach the SCN from retinal receptors which lie particularly in the lower and nasal quadrant of the retina. The receptors are the retinal ganglion cells and their light-sensitive pigment is…

The characteristics of this rhythm generator are largely genetically determined. At least 12 genes have been identified on chromosome 5 which, through inhibiting or promoting protein transcription, form a complex negative…

REM sleep is almost never normally entered from wakefulness except in infants. It is preceded by NREM sleep, usually stages 1 and 2. It can therefore be regarded as a type of arousal…

This includes a variety of mechanisms that influence sleep but which are independent of the time spent awake and of circadian and ultradian rhythms. They modify the sleep–wake cycle according to changes in the environment…

Even before the onset of NREM sleep the heart rate falls in parallel with the reduction in body temperature and is related to the fall in metabolic rate. Vasodilatation…

Homeostatic control of cardiovascular function in response to, for instance, changes in blood volume or temperature is poor in REM sleep. There is considerable variability of heart rate and…

Less saliva is produced during sleep than during wakefulness, possibly partly related to a reduction in oromotor activity, but the volume is similar in NREM and REM sleep. The…

The minute ventilation falls (1.23) and the arterial P_co2  rises by 2–3mmHg. The stage of NREM sleep oscillates frequently at sleep onset and the threshold for carbon dioxide to…