biological rhythms Flashcards
circadian rhythms
Roughly a 24 hour cycle.
Help to optimise a person’s physiology and behaviour to best meet the varying demands of the day / night cycle
For example - the sleep-wake cycle which dictates when we should be sleeping and when we should be awake. Light and darkness are the exogenous zeitgebers that determine when we feel the need to sleep and when we need to wake up.
This circadian rhythm also dips and rises at certain times of the day, so our strongest sleep drive usually occurs in two dips (between 2-4am and between 1-3pm, the ‘post lunch dip’). The sleepiness we feel during these circadian dips is less intense if we had sufficient sleep, and more intense when we’re sleep deprived.
The internal circadian ‘clock’ Is described as free running as it will maintain a cycle of about 24-25 hours, even in the absence of external cues.
The circadian system is intolerant of any major alterations in sleep and wake schedules because this causes the biological clock (and the internal physiological systems that are dependent on this) to become completely out of balance.
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Michel Siffre - lived underground in a cave in Texas for 6 months and had no clues from the sun as to what time of day it was. Found that his sleep/ wake cycle settled into a 25 hour rhythm which shows that our biological clock continues in a cyclical pattern and suggests that our endogenous pacemakers can maintain a regular circadian rhythm in the absence of exogenous zeitgebers.
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Shift work - you are 3x more likely to develop heart disease if you participate in shift work. This is because there is stress being put on circadian rhythms.
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Real world application - chronotherapeutics (the study of how timing affects drug treatments). The specific time a patient takes their medication is very important as it can have a significant impact on treatment success. 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. Researchers found the greatest risk of heart attack is during the early morning hours after waking. Therefore, medications have been developed with a novel drug delivery system, patients take their medication when going to sleep at 10pm, but the actual drug is not released before the vulnerable period of 6am to noon. Therefore research into circadian rhythms has been helpful in developing drug treatments.
ultradian rhythms
More than 1 cycle every 24 hours / last less than 24 hours
An example of an ultradian rhythm is the different stages of sleep - during the night, a sleeping person will typically cycle between five stages: Stages 1 and 2 - where light sleep begins and the person can be easily woken. At the beginning of sleep brainwave patterns start to become slower and more rhythmic (alpha waves), becoming even slower as sleep becomes deeper (theta waves)
Stages 3 and 4 - these stages involve delta waves which are even slower. This is deep sleep or slow wave sleep and it is difficult to rouse someone at this point.
Stage 5 - REM sleep, the body is effectively paralysed yet brain activity speeds up significantly in a manner that resembles the awake brain. REM stands for rapid eye movement to denote the fast jerky activity of the eyes under the eyelids at this point. Research has suggested that REM activity during sleep is highly correlated with the experience of dreaming
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Differences in the sleep patterns of individuals are usually attributed to differences in non-biological factors such as room temperature and sleep hygiene. However, a study suggests that these differences are largely biologically determined.
Participants were studied over 11 consecutive days and nights in a strictly controlled laboratory environment. The researchers assessed sleep duration, time to fall asleep and the amount of time in each sleep stage. They found large individual differences in each of these characteristics, which showed up consistently across the 8 nights. For deep sleep (stages 3 and 4) the individual differences were particularly significant. This meant that differences between participants were not driven by circumstances, but were at least partially biologically driven.
infradian rhythms
Less than 1 cycle every 24 hours / last more than 24 hours
The menstrual cycle - the average menstrual cycle appears to be around 28 days and is regulated by hormones (endogenous pacemakers) which either promote ovulation or stimulate the uterus for fertilisation. Ovulation occurs roughly half way through the menstrual cycle, when oestrogen levels peak, and usually lasts for 16 to 32 hours. After the ovulatory phase, progesterone levels increase in preparation for the possible implantation of an embryo into the uterus.
Annual rhythms - in most animals, annual rhythms are related to the seasons (e.g. migration as a response to lower temperatures and decreased food sources in the winter). In some humans an example of an annual infradian rhythm is seasonal affective disorder.
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The menstrual cycle is normally governed by an endogenous system - the release of hormones by the pituitary gland. However, it can also be controlled by exogenous cues. When several women of childbearing age live together and do not take oral contraceptives, their menstrual cycles tend to synchronise.
