Biological Rhythms

Biological Rhythms

Circadian, infradian and ultradian and the difference between these

rhythms

The physiological processes of living organisms follow repetitive cyclical variations over

certain periods of time. These bodily rhythms have implications for behaviour, emotion

and mental processes.

There are 3 types of bodily rhythms:

1. Circadian rhythms: follow a 24-hour cycle: e.g. the sleep-waking cycle

2. Ultradian rhythms: occur more than once a day: e.g. the cycles of REM and NREM

sleep in a single night’s sleep

3. Infradian rhythms: occur less than once a day: e.g. menstruation (monthly) or

hibernation (yearly)

All bodily rhythms are controlled by an interaction of:

1. Endogenous pacemakers (EP’s). Internal biological structures that control and regulate

the rhythm.

2. Exogenous zeitgebers (time givers) (EZ’s). External environmental factors that

influence the rhythm.

Heart rate, metabolic rate, breathing rate and body temperature all reach

maximum values in the late afternoon/early evening and minimum values in the

early hours of the morning. If we reverse our sleep-waking pattern these rhythms

persist. This indicates human bodies are evolved for activity in the day and rest

at night and, indeed, being nocturnal or disrupting the circadian cycle is highly

stressful and physiologically and psychologically harmful.

The EP controlling the sleep-waking cycle is located in the hypothalamus.

Patterns of light and darkness are registered by the retina, travel up the optic

nerves to where these nerves join (optic chiasma), and then pass into the

superchiasmatic nucleus (SCN) of the hypothalamus. If this nerve connection is

severed circadian rhythms become random. The same effect is produced by

damaging the SCN of rats, and people born without eyes cannot regulate bodily

rhythms.

Ralph bred a group of hamsters to follow a (shortened) 20-hour circadian cycle.

SCN cells were removed and transplanted into the brains of rat foetuses with

CIRCADIAN RHYTHMS

normal rhythms. Once born, these rats adopted a 20-hour cycle. Their brains

were then transplanted with SCN cells from 24-hour cycle hamsters and within a

week their cycles had adopted this new 24 cycle.

When cells from the SCN were removed from rats the 24-hour cycle of neural

activity persisted in the isolated cells. Recent research by Yakazaki found that

isolated lungs and livers, and other tissues grown in a lab still persist in showing

circadian rhythms. This suggests cells are capable of maintaining a circadian

rhythm even when they are not under the control of any brain structures and

that most bodily cells are tuned in to following a daily circadian rhythm.

All of this evidence points to the fact that circadian rhythms are primarily

controlled by evolutionarily-determined, biological structures that exert a strong

influence on us to maintain normal sleep-waking patterns.

However, circadian rhythms are also influenced by EZ’s - ‘cues’ in the

environment- about what time of day or night it is. In 1975 Siffre spent 6 months

underground in an environment completely cut off from all EZ’s. Although he

organised his time in regular patterns of sleeping and waking his body seemed to

have a preference for…