branch ANIMALS

=Biological Clock= A biological clock is an internal timing system which continues without any external time cues and controls the activities of plants and animals.A biological clock is only really an advantage if it means the organism can '''anticipate''' environmental changes. Thus the '''biological clocks are set to environmental rhythms''' such as daily, tidal and seasonal changes. Remember though that the behaviour itself is mediated by the endogenous (internal) biological clock which is set to these exogenous (external) environmental cycles. =Biological Clock in Humans= [image:] In humans we have two interacting systems; an electrical (nervous) system and a chemical (homone / endocrine) system. The hypothalamus links the nervous system to the endocrine system. Part of the hypothalamus, known as the '''Suprachiasmatic nucleus''' (S.C.N), controls our endogenous rhythms, this is our biological clock! It sits directly on top of the optic chiasm (where the optic nerves cross) and receives signals from the retina which let it know what time of day it is. These signals act as a Zeitgeber ensuring our biological clock does not get ‘out of sync’. If we fly to another country it is these signal that will entrain our biological rhythm to match the local environment / time zone. The suprachiasmatic nucleus (S.C.N) sends signals via the nervous system to the Pineal gland stimulating it to produce the hormone '''melatonin''', sometimes referred to the ‘sleep hormone’. Melatonin levels are low during the day and peak at night, making us drowsy. ''Zeitgeber (external cues) that entrain the biological clock in humans and some of the behaviours / rhythms they regulate:'' [image:] ''Melatonin & our circadian rhythms:'' [image:]