Circadian rhythms Flashcards
What is a circadian rhythm
This is a biological rhythm that lasts for approximately a day (24 hours)
The sleep-wake cycle
The circadian rhythm dictates both when we should be sleeping and awake
Light and darkness are the external signs, but the circadian rhythm also dips and rises and different times of the day
Strongest sleep drives:
2-4 am
1-3 pm
The sleepiness we experience during these circadian dips is less intense if we have had sufficient sleep and more intense if we are sleep deprived
The internal circadian ‘clock’ will maintain a cycle of about 24 - 25 hours even in the absence of external cues
Core body temperature
This is at its lowest (about 36°C) at about 4:30am and at its highest (about 38°C) at about 6pm
Sleep occurs when the core temperature begins to drop, and body temperature begins to rise during the last hours of sleep, promoting a feeling of alertness in the morning
A small drop in body temperature also occurs in most people between 2pm and 4pm, suggesting why many people feel sleepy in the early afternoon
Hormone production
Hormone release follows a circadian rhythm
The production and release of melatonin from the pineal gland in the brain peaks during the hours of darkness
By activating chemical receptors in the brain, melatonin encourages feelings of sleep
When it is dark, more melatonin is produced, and when it is light again, the production of melatonin drops and the person wakes
Case study - Siffre
He spent two months in the Scarasson underground glacier in southern France
He used a one-way communication system to tell his research team back on the surface the times that he woke, ate his meals and went to sleep, but he received no information from them, and he had to estimate the passage of time on his own
Temperatures were below freezing and humidity was at 98%
Every time he called his research team, Siffre took his pulse, and had to count from 1 to 120, at the rate of one digit per second to assess his judgement of the passing of time
He found that in the absence of exogenous zeitgebers (external cues), his sleep/wake cycle was 24 hours and 30 minutes suggesting that exogenous zeitgebers act as a correcting mechanism, resetting the endogenous pacemakers (body clocks) inside us
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Siffre’s second studies
He conducted two lengthier time-isolation experiments again with himself as the subject
E.g. he spent 205 days at the bottom of Midnight Cave in Del Rio, Texas
Found similar results - the circadian rhythm of his sleep/wake cycle was out of sync with the day-night rhythm of the world
Increased the reliability of his original findings as the results were repeated and found to be consistent across different studies
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Miles et al
He found a 24.9 circadian rhythm in a man who was blind from birth
Even though he was told the time, he struggles to adhere to a 24-hour pattern, and has to rely of sedatives and stimulants in order to cope
Suggests that our circadian rhythms are controlled by out endogenous pacemakers and are less affected by exogenous zeitgebers than Siffre though
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Chronotherapeutics
This is a real world application of circadian rhythms - the study of how timing affects drug treatments
It is essential that the right concentration of a drug is released in the target area of the body at the time that the drug is most needed. For example, the risk of heart attack is greatest during the early morning hours after awakening
As a result, chronotherapeutic medications have been developed with a novel drug delivery system. These medications can be administered before the person goes to sleep at 10pm but the actual drug is not released until the vulnerable period of 6am to noon
Circadian rhythms AO3
Folkard et al
12 participants asked to live in a cave for three weeks, isolated from natural light and other time cues
Told to go to bed when the clock showed 11:45pm and to get out of bed when it showed 7:45pm
Initially the clock was run normally but it eventually sped up until eventually 24 hours was indicated when intact only 22 hours had passed
To begin with, the participants circadian rhythms matched the clock, but then continued to be 24 hours rather than a 22 hour cycle as imposed by the experiment (with the exception of one participant)
Suggests that the human circadian rhythm can only be guided to a limited extent by exogenous zeitgebers and that clocks are less important zeitgebers than sunlight
