exogenous zeitgebers

• Exogenous zeitgebers are external cues that play an important role in regulating biological rhythms helping to synchronise and reset them. Endogenous pacemakers interact with exogenous zeitgebers and this process is known as “entrainment”. The opposite of entrainment is “free-running” and here the biological clock operates without external cues.
• Light is the most dominant zeitgeber and has been found to reset the body’s main pacemaker the Suprachiasmatic nucleus (SCN) as well as other oscillators located throughout the body. 
• Social cues were originally thought to be the main zeitgeber for circadian rhythms as our daily rhythms appeared to be dictated by social convention. We would eat meals, go to sleep and wakeup at times designated appropriate by our age and social factors. Although light has been found to be the main zeitgeber, other cells have been found to react to social cues such as meal times. 
• Temperature has also been found to be an exogenous zeitgeber. An example of this is when trees see their leaves change color or fall off as they react to the temperature around them or the length of the day. Temperature is also an important factor for the onset of hibernation in animals and in the absence of light, temperature may become the dominant zeitgeber that resets biological rhythms.