Circadian rhythms

How long is a circadian rhythm

24 hours

Example of a circadian rhythm

Sleep/wake cycle

Siffre: Procedure

• Stayed in a cave for 2 months

• Deprived of natural light, clock, calendar & sound

• Had adequate food & drink & only slept when his body told him to.

Siffre: Findings

• He resurfaced thinking it was mid september when it was mid august

• His circadian rhythm changed to around 25 hours

• Lack of external cues made him feel a day was longer than it actually was

Aschoff & Weaver: Procedure

• 4 weeks in WW2 bunker

• Participants were shielded from natural light, temperature change & other environmental cues

• Had access to artificial light

Aschoff & Weaver: Findings

• Circadian rhythm of 25 hours - one even displayed 29 hours

• Suggests natural sleep/wake cycle may be longer than 24 hours, but we use natural light to to adjust pacemakers associated with 24 hour clock

Simon Folkard et al: Procedure

• 12 participants, 3 weeks in cave, no light

• Participants would sleep at 11.45pm and wake up at 7.45 am according to the clock provided

• Researchers sped up the clock from 24 hrs to 22hrs

Simon Folkard et al: Findings

• Only 1 adjusted comfortably to new regume

• Suggesting that we have a strong free running circadian rhythm that can’t be overridden easily by changes in external environment

Evaluation AO3 (1)

• Research tends to involve small experimental groups, limiting degree of meaningful generalisations

• Research has provided useful applications for disrupted circadian rhythms e.g night shifts; more prone to making mistakes & accidents more likely, highlighting economic implications

• Individual differences, cycles vary - some prefer sleeping late & early, innate differences can further complicate generalisation

• Still had access to artificial light, assumed that it had no effect but research opposes this idea; a potential confounding variable, decreasing validity

Morgan’s Hamster study: Procedure

• Removed & transplanted SCNs from hamsters

• Hamsters were bred so they had a 20 hour circadian rhythm

• SCN cells from abnormal hamsters transplanted onto brains of normal ones

Morgan’s Hamster study: Findings

• Normal hamsters adopted same circadian rhythm as their donor

• When nocturnal hamsters had their SCNs replaced with ones who weren’t nocturnal, they followed the same pattern as their donor

• Showing significance of SCN & how endogenous pacemakers are important for biological rhythms

Campbell & Murphy: Procedure

• 15 participants woken up at different times & light pad was shone on the back of their knees

Campbell & Murphy: Findings

• Researchers found change in their sleep/wake cycle of up to 3 hours in some cases

• Showed that light may be detected by skin receptors even when light receptors in eyes don’t detect the same

• Suggesting that light is a powerful exogenous zeitgeber that doesn’t need to rely on the eyes to influence the brain

Evaluation AO3 (2)

• Ethical concerns as some studies were done on animals

• Methodological issues - Campbell & Murphy’s study had yet to be replicated; participants may have been exposed to a small amount of light near eyes - a confounding variable affecting validity of results