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Infradian and ultradian rhythms
Infradian rhythms last longer than one day

Ultradian rhythms are shorter than one day

The sleep cycle is an ultradian rhythm

The menstrual cycle and seasonal affective disorder are examples of infradian rhythms

Ultradian Rhythms
These are rhythmic cycles with a period of less than one day. Examples include levels of alertness throughout the day and the cycle of brain activity during sleep.

The sleep cycle:
1. Each cycle lasts for about 90 minutes.
2. The amount of Stage 3 & 4 sleep decreases with each cycle.
3. The amount of REM sleep increases with each cycle.

There are many methodological problems with researching the sleep cycle and the nature of sleep, most of which arise from the assumptions that researchers need to make that, for example, dreaming only occurs during REM sleep, and the subjective nature of dreaming. The link between REM sleep and dreaming is important as it may provide a way for researchers to understand the purpose of dreaming, but it does rely on the erroneous assumption that REM sleep equals dreaming.

Dement & Kleitman (1957) found participants were likely to be dreaming when woken from REM sleep, but many participants did not report dreams from REM sleep, and many reported dreaming outside REM sleep. This questions the assumption that dreaming only, and always, occurs during REM sleep.

Infradian rhythms
Infradian rhythms have a cycle of longer than one day. Examples include the menstrual cycle and seasonal affective disorder.

The menstrual cycle lasts around 28 days, or one lunar month, and has been the subject of psychological research due to the behavioural changes that are often linked to the cycle, such as increase in accidents, lower academic achievement, suicides and crime (Dalton, 1964). Understanding biological rhythms has changed PMS from being a psychological disorder to a physiological event.

An interesting piece of research by Russell et al (1980) found that the menstrual cycle can be synchronized by exogenous cues, which explains the phenomenon that women who live together often have synchronized cycles. To demonstrate this the researchers used volunteer participants who did not live together, and transferred sweat from the upper lips of the participants to each other with the results that their cycles quickly became synchronized. They hypothesized that sweat must contain a pheromone, or chemical messenger, that resets the menstrual cycle.

Seasonal affective disorder (SAD) is an infradian rhythm that lasts about a year. Features of SAD include depression in winter months that improves to normal mood in summer months. SAD is believed to be caused by low light levels in winter, especially as an effective treatment for the disorder is to spend time exposed to bright light from a light box during the winter months, however the effectiveness of light box therapy may be psychological rather than physiological as Eastman et al (1998) found there was a placebo effect in 32% of participants who used light boxes.

Research into biological rhythms is determinist
Researching biological rhythms such as the menstrual cycle has led to a deterministic understanding of behavioural changes associated with pre-menstrual syndrome. The determinist argument has been used in court to defend women against charges of murder (Johnson, 1987), and psychiatrists such as Dalton (an expert witness) have argued that PMS is a severe mental disorder. To counter this, however, there is evidence that biological rhythms can be influenced by free will, for example Born et al (1999) found that people who will themselves to wake up earlier than usual had higher levels of ACTH in their blood (ACTH is part of the waking up cycle).

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