Biological Rhythms and Sleep

  • Created by: Katie
  • Created on: 09-01-11 23:59

Biological Rhythm Cycles vary in length.

1) Circadian Rhythms are cycles that occur once every 24 hours. For example, the Sleep-wake Cycle, Core Body Temperature and Hormone Production.

  • Michel Siffre investigated the effects on the sleep-wake cycle. Aschoff and Wever (1986) places participants in an underground World War 2 bunker, deprived of environmental and time cues, their rhythms settled to about 24 to 25 hours. These studies suggest Circadian Rhythm exists without zeitgebers, demonstrating an internal endogenous 'clock'. However a study done by Czeisler et al. (1999) showed that cycle length varies between 13 and 65 hours.
  • There has been research which suggests changes in body temperature effect cognitive performance. It is lowest at 4:30am (36 degrees) and highest and 6pm (38 degrees). Giesbrecht et al. (1993) places participants in cold water, they then endured several tasks and those that had lower body temperatures did worse on these tasks. Folkard et al. (1977) studied child being read stories at 9am and 3pm. Those who were read to at 3pm had the better long term recall, suggested better cognitive performance when body temperature is highest. However, Hord and Thompson (1983) found no correlation between cognitive performance and body temperature. However there may be other factors not taken into consideration, it may be that higher body temperature increases arousal which increases cognitive performance (Yerkes-Dodson 1908).
  • Hormone Production is a Circadian Rhythm, as cortisol, melatonin and growth hormone are once every 24 hour cycles.

2) Infradian Rhythms have cycles that occur less than once every day. For example, the Menstrual Cycle.

  • The Menstrual Cycle regulates ovulation. The pituitary gland releases hormones, an egg is ripened and oestrogen is released. One third of women develop PMS (Premenstrual Syndrome) the week before ovulation. Symptoms include acne, anxiety, mood swings, depression, fatigue and agression. Russell et al. (1980) studied the syncronisation of women's mentrual cycles. It has been shown that women that live together cycle's syncronise. Sweat of one group of women was placed on the upper lip of another group, and eventually their cycles syncronised. This could be because of pheromones, which are chemicals produced by a person that effect not themselves but people around them.

3) Ultradian Rhythms have cycles that occur more than once every 24 hours. For examples, Sleep Stages.

  • There are 5 Sleep Stages, the cycle goes for 90 minutes. However in research, there is the assumption that REM sleep is dreaming sleep. This link was first suggested by Dement and Kleitman (1957), who woke participants up during REM, and they reported dreaming. However they also reported dreaming in NREM sleep.

Biological Rhythms are regulated by Endogenous Pacemakers and Exogenous Zeitgebers.

1) Exogenous Zeitgebers are external cues which act as a prompt, and may trigger a biological rhythm.

  • Light is the most important zeitgeber. Siffre (1975) spent six months in a cave with no clocks or natural light, and his cycle extended to a 25 to 30 hour cycle. This suggests that natural light is needed to tune…




beautifully summerised thanks :)

muten roshi


really useful. thanks a lot.



This is really clear, thanks so much. The highlighting of studies really helps 



i love this!