Sleep

1. A circadian rhythm is any biological process that displays an endogenous, entrainable oscillation of about 24 hours.




Diagram illustrating the influence of dark-light rythms on circadian rythms and related physiology and behavior.



2. The suprachiasmatic nucleus is a tiny region located in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms.


3. The pineal gland is a small endocrine gland in the vertebrate brain. It produces melatonin, a serotonin derived hormone, which affects the modulation of sleep patterns in both seasonal and circadian rhythms.
4. The hormone, melatonin, is involved in the entrainment (synchronization) of the circadian rhythms of physiological functions including sleep timing, blood pressure regulation, seasonal reproduction and many others.
5. In addition to melatonin, the amount of cortisol present in the blood undergoes diurnal variation; the level peaks in the early morning (approximately 8 a.m.) and reaches its lowest level at about midnight-4 a.m., or three to five hours after the onset of sleep.