Circadian Rhythm

• Circadian rhythms repeat approx once every 24 hrs e.g. sleep wake cycle. They are driven by our body clocks and found in all of the cells of the body. All of it is synchronised by the master circadian pacemaker - the suprachiasmatic nuclei (SCN) in the hypothalamus. The SCN must constantly be reset by external input so that our bodies are in synchrony with the outside world
• Example of circadian rhythm


  Sleep wake cycle - light + darkness are external signals (exogenous zeitgebers) that determine when we feel the need to sleep/wake. SCN responds to increases in light, triggering the penial gland to deplete melatonin leading us to be awake. Dark = increase melatonin = sleep. Also when we have been awake for a long period of time, homeostasis leads to the need for sleep increasing as energy is being used up by wakefulness. the drive for sleep gradually increases through the day reaching max in late evenings as energy depletes we fall asleep.
• Extra example of circadian rhythm - core body temp


  Body temp tends to be at its lowest at about 4.30am (approx 36°C) and its highest is around 6pm (approx 38°C). Lower temp = sleepier than usual