Biological Rhythms (Circadian, Infradian, Ultradian)

Overview – Biological rhythms

The activities of the mind and body follow various cycles, which are known as biological rhythms. These biological rhythms are categorised in the following ways:

Category	Length	Example
Circadian	24 hours	Sleep and wake cycle
Infradian	More than 24 hours	Female menstrual cycle
Ultradian	Less than 24 hours	Stages of sleep

These biological rhythms are controlled by:

Endogenous pacemakers,
which are influenced by exogenous zeitgebers.

Control systems

Biological rhythms are controlled by endogenous pacemakers – internal biological mechanisms that regulate the body’s natural cycles.

However, these rhythms can also be influenced by exogenous zeitgebers – external environmental cues such as light, temperature, and social activities.

Endogenous pacemakers

Endogenous pacemakers are things within the body that regulate biological rhythms (your ‘body clock’).

For example, the suprachiasmatic nucleus (SCN), which is located in the hypothalamus, acts as the body’s main biological clock. It receives information about light levels from the eyes and uses this information to help regulate the sleep–wake cycle. The SCN then influences other biological processes, such as the release of melatonin, which promotes sleep.

Examples of the effect of endogenous pacemakers:

Ralph et al (1990) transplanted cells from the suprachiasmatic nucleus of hamsters with a 20-hour circadian rhythm into normal hamsters with a 24-hour circadian rhythm. This shortened the circadian rhythms of these hamsters to 20 hours from 24, which illustrates the importance of this endogenous pacemaker in maintaining circadian rhythms. However, as an animal study, these findings may not be valid when applied to humans.

Exogenous zeitgebers

Exogenous zeitgebers: Cues in the external environment that inform endogenous pacemakers to regulate biological rhythms. They provide information to the body about the time of day and can help keep our internal biological clock synchronised with the outside world.

For example, light and darkness are important exogenous zeitgebers. Light detected by the eyes sends signals to the suprachiasmatic nucleus (SCN), which helps regulate the sleep-wake cycle. In darkness, the SCN signals the pineal gland to release melatonin, which makes you feel sleepy and tired. In the presence of light, melatonin production decreases, helping us feel more awake.

Examples of the effect of exogenous zeitgebers:

Campbell and Murphy (1998) conducted an experiment where participants were woken up in the middle of the night. After waking, one group had light shone onto the back of their knees whereas the control group went through the same procedure but the lightbulb was unplugged. The researchers observed that the group who’d had light shone on the backs of their knees had a greater deviation from their original circadian rhythm compared to the control group. This demonstrates the importance of the exogenous zeitgeber of light (even when shone on the skin rather than the eyes) for regulating circadian rhythm.

Relative importance

Relative importance: Endogenous pacemakers appear to be more important than exogenous zeitgebers in regulating circadian rhythms.

There are many studies where circadian rhythms remain regular despite significant disturbances to exogenous zeitgebers. For example:

Speleologist Michael Siffre conducted several case studies (using himself as a subject) on the effects of living in a cave without the exogenous zeitgeber of natural light. In 1962, he spent two months in a cave without any natural light and without a clock. Then, in 1975, he conducted a similar experiment but for six months. In both experiments, Siffre maintained a regular sleep/wake cycle and circadian rhythm of around 25 hours.
Aschoff and Wever (1976) conducted an experiment where participants were kept in a World War 2 bunker without any natural light for four weeks. All participants (except one) maintained a circadian rhythm very close to 24 hours, despite the absence of natural light.
Folkard et al (1985) conducted a similar experiment where participants were kept in a cave without sunlight for three weeks. The participants were supposed to go to bed when a clock said 11:45pm and wake when it said 7:45am, but unbeknown to them the researchers slowly increased the clock speed so that what seemed like a 24-hour day was actually only 22 hours. Despite these faster clocks, all but one participant maintained a consistent 24 hour circadian rhythm.

Circadian rhythms

Circadian rhythms are biological cycles lasting approximately 24 hours.

An example of a circadian rhythm is the sleep/wake cycle: You might cycle between sleeping for 8 hours when it gets dark and being awake for 16 hours during the day, for instance.

Examples of endogenous pacemakers that control circadian rhythm include systems that release hormones such as melatonin, systems that regulate body temperature, and systems that control metabolism and digestion. The main system that controls circadian rhythms is the suprachiasmatic nucleus (SCN).

These internal processes are influenced by exogenous zeitgebers – perhaps the most obvious of which is sunlight. For example, the darkness of night is thought to trigger melatonin release, which makes you feel tired and want to go to bed.

Discussion points: Circadian rhythms

Practical applications: Understanding circadian rhythms can help improve the sleep and health of shift workers, for example. Czeisler et al (1982) found that employees whose shifts were stable over 21 days or more had greater employee satisfaction, improved health estimates, and were less likely to leave their job than employees whose work schedules changed every week. This is likely because keeping shift schedules over 21 days or more allowed workers’ circadian rhythms to adjust to their work schedules. This knowledge can be used by businesses that employ workers on shifts to improve employee satisfaction and reduce employee turnover.
Methodological concerns: Much of the research on circadian rhythms is in the form of case studies or experiments using small sample sizes. As such, these findings may not be valid when applied to the general population.

Infradian rhythms

Infradian rhythms are biological cycles lasting more than 24 hours.

An example of an infradian rhythm is the human menstrual cycle: Women typically ovulate once every 28 days.

menstrual cycle infradian rhythm

As with circadian rhythms, infradian rhythms are controlled by endogenous pacemakers. For example, hormones such as estrogen and progesterone are crucial to the menstrual cycle.

Infradian rhythms can also be influenced by exogenous zeitgebers. For example, Stern and McClintock (1998) demonstrated that women’s menstrual cycles change when exposed to pheromones from other women.

Ultradian rhythms

Ultradian rhythms are biological cycles lasting less than 24 hours.

An example of an ultradian rhythm is the different stages of sleep: During the night, a sleeping person will typically cycle between five stages:

Stage	Length	Description
1	5-15 minutes	Light sleep. Alpha waves increase and brain activity starts to reduce. Heart rate slows and muscles relax.
2	5-15 minutes	Light sleep. Brain activity reduces but with occasional bursts of activity.
3	5-15 minutes	Deep sleep. Delta brain waves increase and brain activity is greatly reduced.
4	~40 minutes	Deep sleep. Delta waves peak, lowest level of brain activity during the sleep cycle.
Rapid eye movement (REM)	>15 minutes	High level of brain activity. Dreams are likely to occur. Body is completely relaxed.

One complete sleep cycle through all these stages will typically take around 90 minutes. So, during a full night’s sleep, a person may repeat this cycle four or five times.