Endogenous pacemakers & Exogenous zeitgebers

Internal body clocks that regulate many of our biological rhythms (i.e SCN & the sleep/wake cycle)

Animal studies involving the SCN have shown that if the SCN connections are destroyed, the animals no longer have a sleep/wake cycle. Ralph et al found that hamsters who received SCN cells through transplant from other hamsters bred to have a 20-hour sleep/wake cycle themselves defaulted to a 20-hour sleep/wake cycle.

The pineal gland and melatonin: The SCN passes information about daylight to the pineal gland, which is located behind the hypothalamus. This gland increases melatonin production, which induces sleep and is inhibited when a person is awake. Melatonin is a possible cause of seasonal affective disorder.

External factors that affect or entrain (reset our biological clocks) our biological rhythms such as the influence of light on the sleep/wake cycle.

Light:  A key zeitgeber as it resets the SCN - so has a key effect on the sleep/wake cycle. Hormone production & other processes are also influenced by light. Campbell and Murphy (1998) found that skin can detect light- when participants had light shone on to the back of their knees, this affected the duration of their sleep/wake cycles - even if it was dark outside. This suggests that light is perceived not just by the eyes, and has an effect on the body.

Social cues: Infants have no set sleep/wake cycle until about 6 weeks of age, & this process generally continues until around 16 weeks, when babies are entrained. This could be due to the schedules imposed on them by parents. Similarly, the effects of jet lag can be reduced by quickly adapting to local times for sleeping and eating (not going to bed when you feel tired, for example). This suggests that the body does respond to cues in the environment.