what is an exogenous zeitgeber

• Exogenous zeitgebers are any factors in the sleep-wake cycle which act as external cues to either wakefulness or sleepiness 
• going to bed with the light on or a digital device placed next to the bed may keep levels of light unnaturally high and thus impact negatively on sleep
• trying to sleep during the day - even if the person is very tired - may not happen due to light levels
• someone may sleep through their alarm clock if it has been set to an unusually early time (their body is not used to waking at this time so their sleep may be very deep)
• Exogenous zeitgebers are responsible for resetting the biological clock of an organism
• • Light is the main and most important exogenous zeitgeber, in the same way as it is crucial to the functioning of endogenous pacemakers
• • Social cues also act as exogenous zeitgebers e.g.
  • the timing of meals (e.g. 'We eat dinner at 7pm every evening')
  • work schedules (e.g. 'I get to work at 8.30am and leave at 4.30pm)
  • exercise sessions (e.g. 'Five-a-side football happens every Tuesday at 6pm')
• People can compensate when there is a lack of natural light using these social cues instead
• • Synchrony occurs between the internal clock (endogenous pacemakers) and external cues (exogenous zeitgebers) e.g.
  • rising levels of light in the morning which triggers wakefulness
  • the alignment of behaviours such as waking, eating meals, working etc. to a regular sleep-wake cycle is known as entrainment
• • Thus, endogenous pacemakers and exogenous zeitgebers interact and work alongside each other to set circadian rhythms