circadian rhythms

• biological rhytms are distinct body patterns that conform to cyclical time periods. these can be influenced by either internal body clocks, or external changes to the environment
• a circadian rhythm lasts 24 hours, an example is the sleep/wake cycle.
• these can be regulated by either endogenous pacemakers or exogenous zeitegebers
• endogenous pacemakers are the body's internal biological body clock, this is called the supra-chiasmatic nucleus which sits above the optic chiasma.
• exogenous zeitegebers are external cues that regulate the body's internal biological clock. an example would be 'light'. this can reset the body's main endogenous pacemaker the SCN, thus it plays a significant role in the sleep/wake cycle.
• the sleep/wake cycle is an example of a circadian rhythm, this is 24 hour cycle that regulates sleepiness and alertness.
• during the day, light is perceived by the retina in the eye and sends a message to the SCN which detects the light. the SCN instructs the pineal gland to inhibit the production of melatonin. this is why we stay awake.
• during the night, darkness is perceived by the retine in the eye, which then sends a message to the SCN which detects darkness. the SCN then instructs the pineal gland to produce melatonin, which makes us feel tired.
• ralph (1991): created a genetically mutated hamster that had an SCN that lasted 20 hours rather than 24 hours. they transplated the SCN into normal hamsters and found that the hhamsters rhythm changed from 24 hrs to 20 hrs.
• folkard (1996): uni student kate aldcroft was housed in a lab for 25 days. she was told to play amazing grace on the bagpipe twice a day at the same time e.g. 9am and 9pm. it was found that the time she played became later and later, and she began to sleep for 16 hrs and her sleep/wake cycle changed to 30 hrs.