Circadian, Infradian and Ultradian Rhythms Revision Notes

Circadian Rhythms

A circadian rhythm is one that lasts for 24 hours. An example of this is the sleep-wake cycle.

·         Temporal Isolation Studies

o    these provide us with a picture of the free-running circadian rhythm and demonstrates that the cycle persists despite the absence of light.

o    Michael Siffre (1975) spent long periods in a cave with no exogenous zeitgebers. He found that his sleep-wake cycle generally adjusted to a 24 hour cycle yet sometimes changed.

o    Aschoff and Wever (1976) placed participants in an underground bunker, the participants’ cycles tended to be around 24-25 hours long.

·         Cycles can be Entrained

o    they can be brought into phase to some extent by external cues.

o    Folkard et al (1985) reduced participants circadian rhythms using a quickening clock but at 22 hours the participants own rhythms took over again showing a limit for control of internal rhythms.

·         Core Body Temperature

o    lowest at around 4:30AM (36’c) and highest at around 6:00PM (38’c).

o    Folkard et al tested memory recalls in children and the results were better when temperature was highest.

·         Post-Lunch Dip

o    tiredness in the afternoon appears to be due to the effects of having eaten, but it also occurs even if you don’t eat due to a circadian temperature rhythm.

·         Hormones

o    Cortisol levels are lowest at midnight, and peak at about 6:00AM – high levels mean increased alertness.

o    Melatonin and growth hormone levels peak at night.

·         Blind Sufferers

o    Miles et al (1977) found that blind people show the same pattern as sighted people therefore it must be due to an internal mechanism.

AO2 EVALUATIVE POINTS

·         Individual Differences

o    There are individual differences in the variation of cycle length.

o    Cseizler (1980) stated that cycle length varies from 13 hours to 65 hours.

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