Biological Rhythms; Circadian, Infradian and Ultradian.

The role of endogenous pacemakers is to set the free-running internal rhythm. It is an internal biological ‘clock’ that allows organisms to control their internal rhythms and helps animals to anticipate cyclical events (e.g., the coming of night). These are innate. The SCN is the endogenous pacemaker that controls the circadian sleep/wake cycle. The SCN sends signals to the pineal gland, directing it to increase melatonin production at night. Melatonin induces sleep by inhibiting brain mechanisms that keep us awake. The SCN therefore maintains the link between light and melatonin production.