Biological Rhythms A01 Flashcards
Definition of biological rhythm (can begin a 16 marker)
The physiological processes of living organisms follow repetitive cyclical variations over certain periods of time. These bodily rhythms have implications for behaviour, emotion and mental processes.
Circadian rhythms
Circadian rhythms: follow a 24-hour cycle: e.g. the sleep-waking cycle
● Body temperature is another circadian rhythm. Human body temperature is at its lowest in the early hours of the morning (36oC at 4:30 am) and at its highest in the early evening (38oC at 6 pm). Sleep typically occurs when the core temperature starts to drop, and the body temperature starts to rise towards the end of a sleep cycle promoting feelings of alertness first thing in the morning.
Ultradian rhythms
Ultradian rhythms: occur more than once a day: e.g. the cycles of REM and NREM sleep in a single night’s sleep
Infradian rhythms: occur less than once a day: e.g. menstruation (monthly) or hibernation (yearly)
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● A monthly infradian rhythm is the female menstrual cycle, which is regulated by hormones that either promote ovulation or stimulate the uterus for fertilisation. Ovulation occurs roughly halfway through the cycle when oestrogen levels are at their highest, and usually lasts for 16-32 hours. After the ovulatory phase, progesterone levels increase in preparation for the possible implantation of an embryo in the uterus. It is also important to note that although the usual menstrual cycle is around 28 days, there is considerable
variation, with some women experiencing a short cycle of 23 days and others
experiencing longer cycles of up to 36 days.
● A second example of an infradian rhythm is related to the seasons. Research has found
seasonal variation in mood, where some people become depressed in the winter, which is known as seasonal affective disorder (SAD). SAD is an infradian rhythm that is governed by a yearly cycle. Psychologists claim that melatonin, which is secreted by the pineal gland during the night, is partly responsible. The lack of light during the winter months results in a longer period of melatonin secretion, which has been linked to the depressive symptoms.
Endogenous pacemakers Internal biological structures that control and regulate the rhythm.
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● The most important endogenous pacemaker is the suprachiasmatic nucleus, which is closely linked to the pineal gland, both of which are influential in maintaining the circadian sleep/wake cycle.
● The suprachiasmatic nucleus (SCN), which lies in the hypothalamus, is the main endogenous pacemaker (or master clock). It controls other biological rhythms, as it links to other areas of the brain responsible for sleep and arousal. The SCN also receives information about light levels (an exogenous zeitgeber) from the optic nerve, which sets the circadian rhythm so that it is in synchronisation with the outside world, e.g. day and night.
● The SCN sends signals to the pineal gland, which leads to an increase in the production of melatonin at night, helping to induce sleep. The SCN and pineal glands work together as endogenous pacemakers; however, their activity is responsive to the external cue of light.
Exogenous zeitgebers (time givers) (EZ’s)- External environmental factors that influence the rhythm
The most important zeitgeber is light, which is responsible for resetting the body clock each day, keeping it on a 24-hour cycle.
● The SCN contains receptors that are sensitive to light and this external cue is used to synchronise the body’s internal organs and glands. Melanopsin, which is a protein in the eye, is sensitive to light and carries the signals to the SCN to set the 24-hour daily body cycle. In addition, social cues, such as mealtimes, can also act as zeitgebers and humans can compensate for the lack of natural light, by using social cues instead.
