Endocrine cycles
There is an important circadian pattern to the secretion of hormones, many of which are controlled by the hypothalamus _[19].

Those needed to adapt the individual to respond to external influences during wakefulness, and which are predominantly catabolic, such as adrenaline and cortisol, are secreted especially during the day, and have a strong circadian influence.

Corticotrophin-releasing hormone (CRH)
This controls release of ACTH. It promotes wakefulness, and inhibits NREM sleep.

Adrenocorticotrophic hormone (ACTH)
The release of ACTH is controlled both by CRH and by arginine vasopressin which also acts on osmoreceptors in the preoptic area of the hypothalamus. Its secretion is inhibited by stages 3 and 4 NREM sleep and changes only slowly with alterations of the timing of circadian rhythms, induced, for instance, by time zone changes (jet lag).

Cortisol
This is secreted in response to ACTH. Its release is closely related to the circadian rhythm but is also slightly influenced by light exposure. Cortisol secretion occurs in several pulses towards the end of the nocturnal sleep episode and peaks around the time of wakening. Its secretion is inhibited by stages 3 and 4 NREM sleep and it also inhibits stages 3 and 4 NREM sleep. In Cushing’s syndrome and psychotic depression cortisol secretion occurs continuously throughout sleep and wakefulness, and the dip in secretion at night is absent.