Circadian Rhythms

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Circadian Rhythms


> Circadian rhythms are those that last twenty-four hours & the two best known ones are the sleep-wake cycle and the body temperature cycle.

The sleep-wake cycle

> The sleep-wake cycle - you feel sleepy when it's dark & are roused by sunlight - or perhaps it's governed by knowing what time of day it is (these are all external cues & are important in guiding the circadian cycle).

> There's also an internal (endogenous) 'clock' & this circadian clock is free running (works without any external cues, setting a cycle of about 24 hours.

> Under normal circumstances the clock doesn't work alone, there are external cues (daylight) that help adjust the internal clock to the environment you live in.

> Studies show circadian rhythms persist despite isolation from natural light, which demonstrates the existence of an endogenous 'clock' - this research also shows that external cues are important because the clock wasn't accurate (varied daily).

Core body temperature

> The core body temperature is one of the best indicators of the circadian rhythm - lowest at about 4.30am (36*c) and highest about 6.00pm (38*c).

> There's a slight dip in body temperature just after lunch which isn't just due to the effects of eating lunch - the dip occurs even when people haven't eaten.


> Hormone production also follows a circadian rhythm.

> Cortisol (hormone produced when we're stressed but is also related to making us alert when we wake up) is lowest around midnight and peaks at around 6am & can explain why, if we wake up at 4am, it's hard to think clearly (levels aren't high enough).

> Melatonin (which induces sleepiness) & growth hormone are 2 other hormones that have a clear circadian rhythm, both peaking at around midnight.


The sleep-wake cycle

Research methodology - 

> Early research studies suffered a flaw when estimating the free-running cycle of the human circadian rhythm (participants were isolated from daylight but not artificial light as they thought it would have no effect).

> CZEISLER ET AL. altered participants' circadian rhythms down to 22 hours and up to 28…


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