All living organisms experience rhythmic changes, which tend to coincide with seasonal or daily environmental changes. These rhythms are known as biological rhythms, which include circadian, ultradian and infradian types.
All biological rhythms are controlled by two different factors - internally (endogenous) through nature, and externally (exogenous) through nurture. Most organisms have internal biological clocks, called endogenous pacemakers. The main endogenous pacemaker in circadian rhythms is the suprachiasmatic nucleus (SCN), a small bundle of nerves in the hypothalamus, as suggested by Morgan (1995), and Kalat (1998). Kalat suggested that low levels of light lead to an electrical stimulant, which activates the pineal gland in the SCN, located in the centre of the brain, to secrete a hormone called melatonin, which causes sleepiness. Its production of melatonin varies with periods of light and darkness in the environment, and it obtains this information about light in the environment by means of nerve pathways originating in the eyes. Therefore the main exogenous zeitgeber ('time-giver') that controls circadian rhythms is therefore light.
Research that suggests the SCN is the main endogenous factor for circadian rhythms comes from Morgan in 1995. His aim was to determine whether the SCN in hamsters is linked to the dissappearance of their circadian rhythms. He removed the SCN from hamsters and transplanted an SCN from mutant hamsters whose biological rhythms have shorter cycles than those of the recipients. He found that their circadian rhythms had disappeared (when the SCN was removed). However, this study can be questioned as the experiment uses hamsters, and it is therefore unreliable to generalise to humans.
Research showing that light is the dominant zeitgeber comes from Miles et al. in
