Biological rhythms

Rhythms that last around 24 hours are known as circadian rhythms.

The sleep-wake cycle is governed by an internal (endogenous) pacemaker - a biological clock called the suprachiasmatic nucleus (aka SCN)

The SCN lies just above the optic chiasm which provides information from the eye about light. 

Exogenous zeitgebers (light) can reset the SCN.

Infradian rhythms take longer than 24 hours to complete for example - the menstrual cycle & seasonal affective disorder (SAD)

The menstrual cycle takes place over around 28 days (this varies between women.) Hormone levels rise during the cycle, which causes the release of an egg (ovulation), then progesterone is released which thickens the womb lining to ready the body for pregnancy. If no pregnancy occurs, the womb lining comes away, resulting in menstruation.

SAD: Seasonal affective disorder (SAD) is a seasonal depressive disorder where sufferers feel a lowered mood, & lowered activity levels, during winter when daylight is shorter. Their mood changes throughout the year - aka - a circannual rhythm. This is thought to be caused by the hormone melatonin, which has a knock-on effect on the production of serotonin, a neurotransmitter linked with depression. More melatonin is released during the winter, as it is released when there is a lack of daylight.

A type of biological rhythm that takes place more than one cycle in 24 hours. For example the sleep cycle:

Stages 1 & 2: Light sleep where a person can be woken up easily. Stage 1 -brain waves are high frequency & have short amplitude - known as 'alpha waves.' Stage 2 - the alpha waves continue but there are occasional random changes in pattern called 'sleep spindles'. 

Stages 3 & 4: known as deep sleep or slow-wave sleep (SWS) where the brain waves are 'delta waves' with lower frequency & higher amplitude. It's difficult to wake someone at this point.

Stage 5: REM sleep - the body is paralysed yet brain activity closely resembles the awake brain - during this time the brain produces 'theta waves' & the eyes occasionally move around (rapid eye movement - REM). Dreams are most often experienced during REM sleep but may also occur in deep sleep.