Circadian rhythms have approximately a 24 hour duration. For example the sleep/wake cycle
Ultradian rhythms have less than 24 hour duration. For example sleep stages
Infradian rhythms have greater than 24 hour duration. For example the menstrual cycle and SAD
How the sleep/wake cycle is controlled: Endogenous pacemakers:
Suprachiasmatic nucleus (SCN) an example of an endogenous pacemaker, found in Hypothalamus.
Pineal gland: Receives information from the SCN to release the hormone melatonin which induces sleep.
Adrenal gland: Releases cortisol which induces wakefulness.
Exogenouszeitgebers; are external factors which are in our environment which can influence our sleep/wake cycle through entrainment, such as light, which suppresses the release of melatonin from the pineal gland.
Supporting evidence for the circadian rhythm
P - evidence for circadian rhythms.
E - Siffre spent extended periods underground to study effects on his biological rhythms. He stayed in a cave for 2 months in the Alps and spent 6 months in a cave in Texas.
E - In both cases, in the absence of natural light, the length of the sleep/wake cycle has extended to 25 hours
L - This shows that endogenous pacemakers must interact or entrain with exogenous zeitgebers to maintain a 24 hour sleepwake cycle
Contradictory evidence for the circadian rhythm
P - Siffre's study was a case study
E - Whilst Siffre did get lots of data about his body and the changes that happened to him whilst in the cave. It relies on a single persons experience as to what happened.
E - cannot be generalised to the other people, as individual cases only provide evidence for that unique case rather than the whole population
L - case studies do provide in depth into the effect of circadian rhythms and how they are different, they do not allow for generalisations of circadian rhythms to the general population