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Infradian rhythms happen less than once a day. They have been known about for centuries. A
seventeenth-century doctor, Sanctorius, weighed healthy young men over long periods of time and
found a monthly weight change of 1-2. Infradian rhythms include annual cycles (cricannual
rhythms), the menstrual cycle and animal activity cycles governed by the moon.
Many animals follow cycles that are linked to moon phases. These enable animals to prepare
physiologically and behaviourally, in advance of changes in their environment, so are adaptive in
A variety of sea creatures have breeding cycles that are linked to moon phases. These lunar cycles
are adaptive as high and low tides are governed by the moon. The Grunion fish is washed onto the
beach each spring tide (the highest high tide in each lunar month, occurs around the time of the full
and new moon). The fish then deposits its sperm and eggs onto the beach, where they are safe (at
least from predators in the sea). The fertilised eggs stay in the damp, warm sand, where they grow.
At the next spring tide, the young fish are sufficiently developed to swim when they are washed
back into the ocean.
Annual rhythms are also shown by many animals, including behaviours such as breeding,
aggression and migration. An example of this can be seen by studying hibernation. Pengelly &
Asmundson (1974) studied the golden-mantled ground squirrel. They looked at the behaviour of 5
squirrels that had been born in captivity and kept in constant darkness. Their environment was
kept at constant temperature and ample food was continuously available. This ensured that cues
about the seasons, available in the wild, were absent. Nevertheless, the ground squirrels continued
to hibernate at roughly the same time each year and emerge at about the same time as they would
if they had been in the wild. This suggests they have an effective long-term clock.
They also found that woodchucks reversed their normal rhythm of hibernation when moved from
USA to Australia as the seasons were at different times. This supports the idea of exogenous
influences. There is, however, research to support the role of endogenous factors in hibernation.
They also found that if blood from a hibernation ground squirrel was injected into a
non-hibernating one, it triggered hibernation. This suggests that a biological factor is present in the
blood that controls this Infradian rhythm.
THE MENSTRUAL CYCLE
Everyone experiences several endocrine cycles. Most however, are difficult to study. The most
marked and well researched is the menstrual cycle in women. The menstrual cycle typically occurs
over a period of 28 days. Based on this average length it is described as circalunar, as although it is
not linked to moon phases, it does correspond with the lunar cycle.
It is controlled by hormones which allow the release of eggs and preparation of the uterine lining
for potential conception. It also has cognitive and emotional impacts. The onset of the menstrual
cycle is irregular at first, but soon becomes well established. It is obviously predominantly governed
by endogenous factors; however, it can be affected by many external cues and events in the
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Reinberg (1967) studied the menstrual cycle of a woman after she had spent 3 months in a cave
with dim lighting. Her sleep/wake cycle lengthened slightly and her menstrual cycle became shorter.
It took a year for this to return to normal. This would indicate that light can affect the menstrual
cycle, probably by affecting the action of the SCN and circadian cycle, which then has implications
for Infradian rhythms.…read more
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For example, PMS has been used as a legal defence. Ms English drove her car into her married lover
after an argument, killing him. She was charged with murder but ultimately was placed on
probation because it was argued in court that her actions were related to severe PMS (Johnson,
1987). It is argued that severe PMS could be considered a mental disorder and therefore individuals
should not be held responsible for their actions. This suggests that biological rhythms may be
beyond our control.…read more