The circadian process
Circa=about , dian=day
The suprachiasmatic nucleus (SCN) dictates the sleep-wake cycles over a roughly 24-hour period
the SCN receives inputs via the retino-hypothalamic fibres
is influenced by external cues (e.g daylight)
but the rhythm is still present in volunteers kept in caves or in people blind from birth
Light initiates sleep in nocturnal animals & waking in diurnal animals (including humans)
Melatonin & melanopsin may play a role
Phase shifts can occur with rapid crossing of time zones
resulting in ‘jet lag’