Biological Rhythms are dictated by two things; the internal body clock (endogenouspacemakers) and external changes in the environment (exogenouszeitgebers)
Some of these rhythms occur multiple times a day (ultradian); some take longer than a day to complete (infradian)
Circadian rhythms are those that last around 24 hours, for example: sleep wake cycle and core body temperature
The Sleep-Wake Cycle
Light, which is an exogenous zeitgeber, is an important factor in the sleep-wake cycle; we feel tired at night and alert during daylight
However, there is also a endogenous pacemaker that influences the cycle - the suprachiasmaticnucleus, just above the opticchiasm, which provides information from the eye about light
Exogenous Zeitgebers can also reset the SCN
Siffre's Cave Study
He has spent several periods underground to see the effects on his own biological rhythms, deprived of sound or light
In each case, his 'free-running' biological rhythm settled down to 25 hours, but he still did fall asleep and wake up on a regular schedule
Aschoff & Weaver's Studies
Had a group of participants spend 4 weeks in a WW2 bunker, deprived of natural light
All but one of the participants displayed a circadian rhythm between 24 to 25 hours
Together with Siffre, this suggests that the natural sleepwake cycle is longer than 24 hours, but it is affected by exogenouszeitgebers (number of daylight hours, typical mealtimes)
Simon Folkard's study
Studied a group of 12 people who agreed to live in a dark cave for 3 weeks; they retired to bed at 11.45pm and woke up at 7.45
Over the course of the study, researchers sped up the clocks (unknown to participants)
The normal 24 hour days only then lasted 22 hours
This then suggests that a strong 'free-running' circadian rhythm cannot be easily affected by exogenouszeitgebers