Been reading about biological rhythms for less than a minute and I’m already confused…

These are rhythms lasting ‘about one day’. The best example of a circadian rhythm is the sleep-wake cycle, associated with which are many cyclical changes with active and dormant periods, for example body temperature and urine production. These rhythms allow animals to prepare for predictable daily environmental changes, such as night and day.
Research has involved participants being deprived of possible zeitgebers (‘time-givers’) like sunrise and sunset, temperature changes during a 24 hour period and wristwatches! Participants tend to maintain a cyclical rhythm but it extends to about 25 hours (Siffre, 1975).
So, endogenous pacemakers can keep a rhythm but exogenous zeitgebers are needed to stick to a 24 hour rhythm.
Where is the brain’s internal clock?
The suprachiasmatic nucleus (SCN) in the hypothalamus is a bundle of nerves with an inbuilt circadian rhythm. This is a particularly important endogenous pacemaker. Evidence for this comes from studies in which the SCN has been cut in hamsters to result in disrupted circadian rhythms (Menaker et al., 1978).
What keeps the brain to a 24 hour rhythm?
Without light the brain’s day would be 25 hours long (a free-running clock). Light is a very important zeitgeber - flashes of light are enough to ‘reset’ the internal clocks of animals living in the dark (Aschoff, 1979). One blind man needed to take stimulant and tranquilizing drugs to maintain a 24 hour cycle!
How does light ensure mammals maintain a 24 hour rhythm?
The pineal gland in the brain converts the neurotransmitter serotonin into the hormone melatonin.
Melatonin is released into the blood stream and causes rhythmic changes around the body. Although the need for sleep is not affected by light, melatonin plays a role in the coordination of the sleep-wake cycle.
(Words are too big and confusing - I hope this doesn’t come up on a test.)
