Biological Rhythms

• A biological rhythm is a change in the body processes or behaviour in response to cyclical changes within the environment
• All biological rhythms are regulated and governed by 2 things: endogenous pacemakers (internal factors) and exogenous zeitgebers (external factors)
• Biological rhythms are cyclical changes in the way that biological systems have evolved because the environment in which organisms live has cyclic changes, day/night, summer/winter.
• The most important rhythms are the circadian rhythms - any cycle that lasts about 24 hours
• Nearly all organisms possess a biological representation of the 24 hour day. These circadian rhythms (often referred to as the body clock) optimize an organism's physiology and behaviour to best meet the varying demands of the day/night cycle
• Our circadian rhythms are driven by our body clocks, found in all the cells of the body and synchronised by the mater circadian pacemaker - the suprachiasmatic nuclei (SCN), found in the hypothalamus.
• The SCN pacemaker must constantly be reset so that our bodies are in synchrony with the outside world
• Light provides the primary input to this system (exogenous zeitgeber), setting the body clock to the correct time in a process called photoentrainment.
• In mammals, light-sensitive cells within the eye act as brightness detectors, sending messages about environmental light levels direct to the SCN. The SCN then uses this information to coordinate the activity of the entire circadian system.
• The sleep-wake cycle dictates when we should be sleeping and when we should be awake. Light and darkness are the external signals that determine when we feel the need to sleep and when we need to wake up.
• The circadian rhythm dips and raises at certain times of the day, so our strongest sleep drive usually occurs in 2 dips (between 2-4am) and (between 1-3pm 0 post lunch dip). The sleepiness we feel during these circadian dips is less intense if we have had sufficient sleep, and more intense if was are sleep deprived.