aqa  A2 Only: Biological rhythms: circadian, infradian and ultradian and the difference between these rhythms. The effect of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle.

Biological rhythms are classified as circadian, infradian and ultradian. A circadian rhythms is one lasting ‘about one day’, such as the sleep-wake cycle, body temperature or urine production. These rhythms allow animals to prepare for predictable daily environmental changes, such as night and day. Infradian rhythms are those which take place over longer than a day (i.e. monthly or seasonally) such as the menstrual cycle, hibernation and seasonal affective disorder. Ultradian rhythms are those shorter than a day, such as REM/nREM cycles in sleep and a proposed ‘Basic-Rest Activity Cycle’, of which research remains inconclusive.

Research is interested in whether these cycles are controlled by internal mechanisms (endogenous pacemakers) or external cues, such as light and weather (exogenous zeitgebers). A whole host of interesting studies have been conducted, which try to isolate endogenous pacemakers, by controlling and manipulating the external environment. Research involving human participants has focused on depriving them of possible zeitgebers (‘time-givers’) like sunrise and sunset, temperature changes during a 24 hour period and wristwatches! Participants tend to maintain a cyclical rhythm but it extends to about 25 hours (Siffre, 1975). So, endogenous pacemakers can keep a rhythm but exogenous zeitgebers are needed to stick to a 24 hour rhythm.

Endogenous Pacemakers & Exogenous Zeitgebers

The main endogenous pacemaker, which has been identified as controlling circadian rhythms, is the suprachiasmatic nucleus (SCN), located in the hypothalamus. It is a bundle of nerves with an inbuilt circadian rhythm. Scientists who isolated cells from the SCN discovered that they beat to a roughly 24 hour rhythm, even outside of the human body. The SCN is connected to the optic nerve and is therefore directly affected by light levels (an exogenous zeitgeber) When light hits the retina of the eye, an action potential sends the message along the optic nerve, which subsequently regulates melatonin secretion form the pineal gland. The pineal gland in the brain converts the neurotransmitter serotonin into the hormone melatonin. Light therefore inhibits the release of melatonin; a hormone which makes you feel sleepy.SCN

Interesting, human case studies into circadian rhythms in the absence of exogenous zeitgebers; particularly light (and therefore a free-running clock) indicate that the brain’s day would be more like 25 hours long. Light is a very important zeitgeber – flashes of light are enough to ‘reset’ the internal clocks of animals living in the dark (Aschoff, 1979). One blind man needed to take stimulants and tranquilizing drugs to maintain a 24 hour cycle!

Biological Rhythms

A circadian rhythm repeats every 24 hours. Human examples include the sleep-wake cycle, urine production, body temperature, etc. There has been some incredibly interesting research into circadian rhythms, both on humans (case studies and small sample sizes) and animals (using more invasive techniques, such as brain lesions).

Michel Siffre –   Over the past 40 years or so he has regularly spent extended periods of time in various caves around the World and agreed to be studied during the process.  His first stint was in 1962 when he spent 61 days in a cave in the Alps.  He emerged on September 17th but thought it was August 20th!  (Note: if you add this up it simply doesn’t make sense.  This means he had lost 28 days which is more than 1 hour a day!).  In 1972 he was monitored by NASA in the caves of Texas and in 1999 he missed the millennium celebrations in a cave in some part of the World.  Each time his body clock extended form the usual 24 to around 24.5 hours. This appears to suggest two things:

  1. There is internal control of the circadian rhythm, since even in the absence of external cues we are able to maintain a regular daily cycle.
  2. There must usually be some external cue that keeps this cycle to 24 hours.  When this is removed we adopt this very strange 24.5 or 25 hour cycle.

Aschoff and Wever (1981) carried out a series of free-running studies similar to Siffre’s, using student participants housed in a specially adapted, soundproof, underground bunker, although for a much shorter period of time (three to four weeks). Their results matched Siffre’s; although there were individual differences in the 300 or so participants, sleep/waking cycles in the absence of external zeitgebers extended to between 25 and 27 hours. Combining these studies provides a more reliable picture of what happens in free-running studies.

Morgan (1995) removed the SCN from some hamsters and found that their rhythms ceased.  However, when they received SCN transplants from other hamsters the cycles were re-established.  In a follow up they selectively bred hamsters that had ‘mutant’ circadian rhythms of 20 hours rather than the normal 24 hours. They then transplanted the SCNs of these hamsters into ‘normal’ hamsters, who then shower the ‘mutant’ 20-hour rhythms.

Decoursey et al. (2000) removed the SCN of chipmunks who were then returned to the wild. A group of control chipmunks with intact SCNs were also released into the wild. After 80 days, many more of the chipmunks without SCNs had been killed by predators, because without an SCN they stayed awake foraging and were easier to locate by nocturnal predators.

These are rhythms with a period of greater than a day. The menstrual cycle is an example of an infradian rhythm. Infradian rhythms that occur as a result of seasonal changes, for example, migration and hibernation are called circannual rhythms.

The menstrual cycle has a period of about 28 days, although the timing can vary according to environmental factors. For example, menstrual cycles may become synchronised if women spend a lot of time together, possibly due to the effect of pheromones passing between them.

Circannual rhythms are under the control of body clocks. But, again, light makes them run on time by influencing melatonin levels. It is how much light or the day length, (known as the photoperiod) that provides the useful information about the changes in the seasons; lengthening days mean Spring and Summer are coming and shortening days predict the onset of Autumn and Winter. In humans, the increased levels of melatonin in autumn appears to lead to a form of depression known as SAD (seasonal affective disorder). This can be treated by exposure to a bright light for several hours per day.


A number of animals that experience a combination of long, cold winters and reduced food availability go through periods of reduced feeding and activity, a sort of ‘deep sleep’, called hibernation. This can last up to seven or eight months, and whilst smaller animals such as the dormouse occasionally ‘wake up’ and snack, larger ones such as the bear tend to hibernate continuously for up to eight months, living off fat stores accumulated the previous autumn. Hibernation involves significant changes to bodily functions – for example to kidney function, metabolism, heart rate and circulation. It has been suggested that these changes are guided by hormones, which in turn influence the hibernation processes of preparation, initiation, maintenance, and final arousal.

Some evidence indicates that these hormonal changes are a result of endogenous mechanisms, with the obvious environmental changes to light and temperature associated with periods of hibernation acting as zeitgebers. Such endogenous timers enable animals to anticipate and prepare for the onset of the harsh winter months. Animals which might normally hibernate do not do so if their environment does not give them the appropriate cues. Black bears, for example, inhabit a wide area of North America and only hibernate when resident in the extreme climates of the Northern regions. Captive black bears do not hibernate as long as they have a constant food source, but do become less active, sleep more and eat less when the weather becomes cold.

Although it hasn’t been isolated or analysed, it seems that there is a chemical in the blood of some animals which induces hibernation. This has been called the hibernation induction trigger or HIT. One idea is that this substance becomes active when triggered by zeitgebers (e.g. days becoming cooler and shorter), inducing animals to prepare for hibernation. Dawe and Spurrier (1968) found early clear evidence for endogenous factors in hibernation. They transfused blood from hibernating ground squirrels into awake active ones and noted that within 48 hours these active squirrels began hibernating even though it was spring (a time they would not normally hibernate).

Menstrual Cycle


The clearest infradian rhythm in human behaviour is the monthly menstrual cycle. The menstrual cycle is a series of physical and hormonal changes that prepare a woman’s body for pregnancy. At the beginning of the menstrual cycle, levels of the hormone oestrogen rise, causing the lining of the uterus to thicken. At around the middle of the cycle, ovulation occurs, whereby an egg is released by an ovary. The egg travels down the fallopian tubes to the uterus. If the egg is fertilised it attaches to the uterus and develops; otherwise the thick lining is not needed and it begins to shed in preparation for another chance of pregnancy the following month. This is menstruation, which usually lasts from three to five days. The average menstrual cycle is 28 days from the start of one to the start of the next, although it can range from 21 days to 35 days. The hormones are co-ordinated by the pineal gland, which may be influenced by light levels and by the secretion of melatonin.

McClintock (1988) began a ten-year longitudinal study that involved 29 women 20-35 years, with a history of irregular, spontaneous ovulation. Samples of pheromones were gathered from nine of the women at certain points in their menstrual cycles by placing pads of cotton under their arms. The women had previously bathed without perfumed products and then wore the cotton pads for at least eight hours. Each pad was then treated with alcohol and frozen. These pads were then wiped under the noses of the 20 other women on a daily basis. 68% of the women responded to the pheromones. Menstrual cycles were either shortened or lengthened, dependent on when in the menstrual cycle the pheromones had been collected. The pheromones collected in the early phases of the women’s cycles shortened the cycles of the recipients by speeding up their pre-ovulatory surge of luteinising hormone. Pheromones collected later, during ovulation, lengthened the menstrual cycles by delaying the luteinising hormone surge.

Seasonal Affective Disorder

Some people suffer from a depressive condition called Seasonal Affective Disorder (SAD) during the winter months and recover during the summer. SAD sufferers experience severe symptoms from seasonal changes such as lowering of moods and depression which is thought to be brought about due to less light and increased melatonin production in darkness by the pineal gland. Serotonin is converted into melatonin and low amounts of serotonin (since it is being converted to melatonin) has been linked with chronic depression.

The symptoms of SAD can be reduced in polar regions by sitting patients in front of very bright artificial lights for at least one hour per day.  This lowers the levels of melatonin in the bloodstream which in turn reduces the feelings of depression.  The precise mechanism for this is still unclear, it could be that melatonin (released from the pineal gland) has a direct affect on mood or it could have its influence indirectly through serotonin.  Drugs used to treat depression such as Prozac and other MAOIs (monoamine oxidase inhibitors), appear to work by altering serotonin levels. Terman et al (1998) researched 124 participants with SAD.  85 were given 30 minute exposure to bright light, some in the morning, some in the evening.  Another 39 were exposed to negative ions (a placebo group). 60% of the am bright light group showed significant improvement compared to only 30% of those getting light in the evening.  Only 5% of the placebo group showed improvement. The researchers conclude that bright light administered in this way may be acting as a zeitgeber and resetting the body clock in the morning.

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 use of an electroencephalogram (EEG) can show the electrical activity of the brain. There are different patterns of activity at different times during sleep (Rechtschaffen & Kales, 1968).

In humans, daily cycles of wakefulness and sleep follow a circadian rhythm. However, within the sleep portion of this cycle another type of rhythm, an example of an ultradian rhythm exists (roughly every 90 minutes in humans). These are the five stages of sleep (outlined below). The first four stages are called NREM sleep (non-random eye movements) and the fifth stage is REM sleep (rapid eye movement), so called because of the accompanying movements of the eye beneath the closed eyelids. One sleep cycle goes through all five stages and lasts about 90 minutes. Stages 1 and 2 are light sleep, characterised by a change in the electrical activity of the brain. The awake brain produces a typical pattern called a beta wave. As you become more relaxed, your brain waves become slower and more regular, with a greater amplitude. Stages 1 and 2 sleep are light sleep, whereas stages 3 and 4 are called slow wave sleep (SWS). In these stages it is very hard to wake someone up, though a person is not unconscious and will be aroused by, for example, their baby crying. In SWS most of the body’s physiological ‘repair work’ is undertaken and important biochemical processes take place such as the production of growth hormones. In REM sleep there is fast, desynchronised EEG activity resembling the awake brain. These cycles continue throughout the night with the SWS period getting shorter and REM periods getting slightly longer as the night progresses. Each sleep cycle is about 60 minutes in early infancy, increasing to 90 minutes during adolescence.


Here are some important points to note:

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

Dement and Kleitman (1957) were the first to demonstrate this link. They woke participants up at the times when their brain waves were characteristic of REM sleep and found that participants were highly likely to report dreaming. However they also found that dreams were recorded outside REM sleep and that sleepers, when awoken in REM sleep, were not always dreaming. The importance of the REM/dream link is that it potentially provides a way to identify when someone is dreaming and therefore might provide theorists with a way to explain dreaming. Here is a link to their original journal article (which is quite short!)

You might be asked an application question on the disruption of biological rhythms. The two most common examples are jet lag and shift work, which are outlined in the videos below.

Evaluating Biological Rhythms

Evolutionary Approach: There is a clear evolutionary advantage to the entrainment of the sleep-wake cycle through exogenous zeitgebers. This adaption allows us, and other mammals, to adapt to our changing environment through the seasons, as well as across time zones. This ability increases our survival chances, as seen by Decoursey et al.‘s (2000) study into the link between a damaged SCN and survival in the wild.

Animal Research: There are a number of problems with using animal studies in this area. The first problem is the harm to the animals concerned. However, there is a costs-benefits consideration to research with non-human animals. The costs to the animals (e.g. Decoursey’s chipmunks were made more vulnerable to predators as a result of being used in the study) can be balanced out by the possible benefits of understanding the important role played by the SCN. There is also the issue of generalisation to humans. It is problematic to generalise animal findings direct to human beings because of differences in the biological systems of different species. For example, in reptiles and birds, light acts directly on the pineal gland through the skull, whereas in humans, this process is mediated by the SCN. This questions the value of findings from animal studies. For ethical reasons it is difficult to carry out the same experiments on human beings (i.e. removing the SCN in order to study the effect on circadian rhythms) as this would cause physical harm to human participants.

Real life: Today, sleeping habits in technologically advanced societies are not determined by darkness. We do not normally go to bed at dusk (especially in winter when, in the UK, dusk can be mid-afternoon). Before the invention of the electric light in the late 19th century, sleep/waking for the majority of people would have been determined by light and darkness. Changes in the range of zeitgebers have been relatively recent in human social evolution.

Cultural differences: In some societies it is not possible to use light and dark as zeitgebers. The Inuit in Greenland have periods in the year of 24 hours of light or 24 hours of darkness. Yet they maintain normal 24-hour sleep/waking cycles. For them, social and work habits, rather than light, are used to synchronise the sleep/waking endogenous pacemaker with the outside world.

Nature/Nurture debate: Biological clocks are innate mechanisms with a clear genetic component, which emphasises nature over nurture; it is difficult for us to change our sleep/waking pattern.

Reductionism: Explanations of biological rhythms are also reductionist as they explain them at the lowest level of physiological mechanisms, sometimes underestimating the contribution of higher-level factors such as social zeitgebers.

Brain2Go Further

Articles on Circadian Rhythms by Science Daily

How light from you smartphone impacts your brain and body