BIOLOGICAL RHYTHMS -ENTRAINMENT

=Entrainment= [image:http://i.imgur.com/tUBq7si.png?1] Entrainment is the process by which the endogenous (internal) biological clock is reset or synchronised with exogenous (external) environmental cues. The environmental cues that reset or calibrate a biological clock are known as '''Zeitgebers''' (German for ‘time givers’). It is important to realise that the pattern of behaviour is not a direct result of the environmental cue. Rather, the environmental cues or Zeitgebers simply act to make sure that the biological clock controlling the behaviour does not get ‘out of sync’ with the environment. For instance, the reason why we feel ‘jet lagged’ when we travel, is that it takes a while for our biological clock to be reset (entrained) to the new time zone. Initially our sleep wake cycles, which are determined by an endogenous (internal) biological clock are out of sync with the environment. Exposure to environmental cues such as sunlight eventually helps to reset the underlying clock. ==Free Running== [image:http://i.imgur.com/4F60WxMl.png] No clock keeps perfect time, especially biological clocks. If the Zeitgebers are removed, the biological clock may start to become out of sync with the environment. Without any external cues (Zeitgebers) we say the organism is '''‘Free Running’'''. In the actogram shown all the environmental cues were removed around day 6 (it was placed in constant darkness). As a result, we can see that the period of activity has started to shift slightly each day. This is known a '''Phase Shift'''. The period of activity is starting a little earlier each day (is shifting to the left). The period of the rhythm is no longer exactly 24 hrs. The new '''Free Running Period''' is actually slightly less than 24hrs, causing the period of activity to start a little earlier each day. Humans have a ‘free running period’ of approx 24hrs 11min. Without any external cues most people would wake up a little later each day. When a group of people trapped in a cave for an extended period of time were finally found, they experienced jet lag as they had become ‘out of sync’ with the external environment.