Circadian Rhythms

Biological Rhythms: Distinct patterns of changes in body activity that conform to cyclical time periods. Biological rhythms are influenced by internal body clocks (endogenous pacemakers) and external changes to the environment (exogenous zeitgebers).

Ultradian Rhythms: Rhythms that occur during the day.

Infradian Rhythms: Rhythms that take longer than a day to complete.

Circadian Rhythms: Biological rhythms, subject to a 24-hour cycle, which regulate a number of body processes such as the sleep/wake cycle and changes in body temperature.

Suprachiasmatic Nucleus (SCN): The internal pacemaker that governs the sleep/wake cycle. It lies just above the optic chiasm, which provides information from the eye about light. Exogenous zeitgebers (light) can reset the SCN.

Siffre (1962): Siffre spent several extended periods underground with no natural light or sound but with access to food and drink. He recorded each time that his ‘free-running’ biological clock settled down to around 25 hours, though he continued to fall asleep and wake up on a regular schedule.

Aschoff and Wever (1976): A group of participants spent 4 weeks in a WWII bunker deprived of natural light. All but one participant (whose sleep/wake cycle extended to 29 hours) maintained a circadian rhythm between 24 and 25 hours.

Folkard et al (1985): studied 12 people who lived in a dark cave for 3 weeks, retiring to bed when the clock said 11.45 pm and rising when it said 7.45 am. The researchers gradually sped up the clock so the 24-hour day only lasted 22 hours and only one participant was comfortable to adjust the new regime.

Consequences of Desynchronisation: Night workers engaging in shift work experience a period of reduced concentration around 6 in the morning meaning mistake are more likely. Also, shift workers are 3x more likely to develop heart disease than people who work more typical work patterns.

Medical Treatment: Circadian rhythms coordinate a number of the body’s basic processes such as heart rate, digestion and hormone levels. This has led to the field of chronotherapeutics. For example, aspirin for heart attacks is most effective if taken last thing at night as heart attacks are most likely to occur early in the morning.

Individual Differences: Studies performed are based on a small sample of participants. Czeisler et al (1999) found individual differences in sleep/wake cycles varying from 13 to 65 hours. Duffy et al (2001) revealed that some people have a natural preference for going to bed early and rising early whereas others are owls.

Shifting the School Day: Wolfson and Carskadon (1998) recommend that the school day start a couple of hours later to fit in with the typical teenage chronotype as hormonal shifts mean that getting sleep becomes more difficult and therefore adolescent students tend to be sleepy at the start of the school day.