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.
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.