The ultradian rhythm found in human sleep follows a pattern of alternating REM and NREM sleep, which consists of stages 1-4. This cycle repeats itself about every 90-100 minutes throughout the night. A cycle consists of a progression through the 4 stages of NREM sleep before entering REM. Most of what we know about these stages comes from recording electrical activity in the brain. Each stage has a distinct EEG pattern. As a person enters deep sleep their brain waves slow and the breathing and heart rate deceases. During REM the EEG pattern resembles that of an awake person.
The Basic Rest Activity Cycle
Kleitman (1969) referred to the 90 minute cycle found during sleeo as the basic rest activity cycle (BRAC). However, Kleitman also suggested that this rhythm continues during the day, even when we are awake. The difference is during the day, rather than moving through sleep stages, we move progressively from a state of alertness into a state of physiological fatigue approximately every 90 minutes. Research suggests the human mind can focus for 90 minutes, and towards the end of this time the body begins to run out of resources.
Infradian Rhythms
Biological rhythms that have a duration longer than 24 hours.
Weekly rhythms.
The grouping of 7 days into a unit called a week is common in most areas of the world. There are obvious human differences that conform to weekly cycles. For example, although male testosterone levels are raised at the weekend, the frequency of births at weekends is lower. It is tempting to look for underlying biological cycles that would dictate these differences but the evidence for weekly infradian rhythms in humans is sketchy.
Monthly Rhythms: Menstrual Cycle
There are considerable variations in the length of this cycle, with some women experiencing a relatively short 23-day cycle whereas others have 36-day cycles. The average appears to be around 28 days. The menstrual cycle is regulated by hormones, which either promote ovulation or stimulate the uterus for fertilisation. Ovulation occurs roughly halfway 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 impregnation.
Annual Rhythms
In most animals, annual rhythms are associated with the seasons. But in humans the calendar year appears to influence behaviour regardless of changes in temperature. Research suggests a seasonal variation in mood in humans, especially women, with some people becoming severely depressed during the winter months (SAD). The winter is also associated with an increase in heart attacks, which varies seasonally and peaks in winter. Also most death occurs in January.
Despite empirical evidence to the contrary the belief in an infradian rhythm based on the phases of the moon remains strong.
Despite empirical evidence to the contrary the belief in an infradian rhythm based on the phases of the moon remains strong.
For example many midwives still believe that more babies are born during a full moon than during a new moon, but the statistics show that this is purely subjective association. Likewise, surveys of workers in the mental health professions have shown a persistent belief that the full moon can alter behaviour, yet study after study has failed to consistently find an association between the moon and human psychopathology.