Biological Rhythms; Circadian, Infradian and Ultradian.

Biological Rhythms – Endogenous Pacemakers and Exogenous Zeitgebers

AO1: Definition of Biological Rhythms:

Biological Rhythms are cyclical changes in the way that biological systems behave. Examples of biological rhythms include:
• Circadian Rhythms (a rhythm that occurs once every 24 hours (e.g. sleep/wake cycle).
• Ultradian Rhythms (a rhythm that occurs more than once every 24 hours (e.g. stages of sleep)
• Infradian Rhythms (a rhythm that occurs less than once every 24 hours (e.g. menstrual cycle).

Biological rhythms must be constantly fine-tuned in order to stay in tune with the external world. In order to achieve this internal time givers (endogenous pacemakers – biological clocks) and external time givers (exogenous zeitgebers) have to work together.

AO1: Description of Endogenous Pacemakers (EPS):

The role of endogenous pacemakers is to set the free-running internal rhythm. It is an internal biological ‘clock’ that allows organisms to control their internal rhythms and helps animals to anticipate cyclical events (e.g., the coming of night). These are innate. The SCN is the endogenous pacemaker that controls the circadian sleep/wake cycle. The SCN sends signals to the pineal gland, directing it to increase melatonin production at night. Melatonin induces sleep by inhibiting brain mechanisms that keep us awake. The SCN therefore maintains the link between light and melatonin production.

AO1: SCN – Suprachiasmatic Nucleus and Pineal Gland:

The SCN is made up of a small cluster of neurons (approx’ 10,000) located in the hypothalamus in the brain. The SCN obtains information about the presence of light (an exogenous zeitgeber) from the optic nerve in the eye (even when the eyes are shut; through the eyelids). Morning light activates the SCN, which signals to the pineal gland to decrease the production of melatonin. At night the SCN signals to the pineal gland that light levels have dropped and the pineal gland coordinates the release of melatonin. The SCN therefore keeps the circadian rhythm in synchrony with the external world.

AO3: Research to Support the Role of Endogenous Pacemakers (EP’s):


(1) Point: Key research to support the role of the SCN comes from Morgan (1995). Evidence: Morgan found that if the SCN of a hamster is removed, then their circadian rhythms will disappear completely. Furthermore, it was found that the rhythms can be re-established by transplanting SCN cells from foetal hamsters.

close up of woman holding a hamster
Photo by Rudolf Jakkel on

Elaboration: This research suggests that the biological rhythm of the sleep/wake cycle (a circadian rhythm) is controlled mainly by internal biological factors.

(2) Point: Further research from Morgan supports the role of the SCN as an EP. Example/Evidence: For example, Morgan found that if a hamster is given a transplanted SCN from another animal (e.g., from a mutant strain of hamster with a shorter cycle of 20 hours), it will adopt the same activity patterns as the donor animal. Elaboration: This research suggests that circadian rhythms are in-fact controlled by internal mechanisms (EPs), in particular, the main biological time keeper is the SCN.

Evaluation Tip: When using research studies as evaluation for description/AO1/Theories, think about how you can evaluate your evaluation. For example, in the case above we are using Morgan as a way of offering support for the role of EPs (in particular the SCN) in the control of internal (circadian) rhythms. When you’ve used Morgan to evaluate this research why not think about the strengths and weaknesses of Morgan’s study (this will allow you to discuss whether Morgan is a valid strength for the SCN/role of EP’s). For example, is Morgan’s study scientific/objective – how are these strengths of research? Morgan used hamsters – is this good? Can this research be generalised (extrapolated) to humans? Is this research ethical?

AO1: Description of Exogenous Zeitgebers (EZs):

Any cue that acts as an external time giver. Light is the main exogenous zeitgeber for humans. Their role is to entrain the otherwise free-running biological rhythms to keep the individual in synchrony with the external world (e.g., awake during the day and sleepy at night). Daylight therefore resets the biological clock (SCN) at the beginning of every day. This happens because bright light suppresses the production of melatonin. Temperature and social cues also act as EZs.

Examples of Exogenous Zeitgebers:

(1) Light:
• Receptors in the SCN are sensitive to changing light levels.
• The SCN uses this information to synchronise the activity of the body’s organs and glands.
• Light resets the internal body clock each day keeping it on a 24-hour cycle.
• Rods and cones in the retina of the eye help to detect changes in light.

(2) Social Cues:
• Social cues such as mealtimes may also have a role of a zeitgeber.
• Aschoff et al (1971) showed that individuals are able to compensate for the absence of zeitgebers such as natural light by responding to social zeitgebers.

AO3: Evaluation and key research to support the role of EZs:


(1) Point: Key research to support the role of EZs in controlling our biological rhythms comes from Michel Siffre (1975). Example/Evidence: For example, Siffre carried out an isolation study in which he spent 6 months in a cave with no external cues (EZs). It was found that his biological sleep/wake rhythm extended (he operated on a 25-hour day). Elaboration: This shows that although sleep/wake cycles are mainly controlled by Eps, EZs are essential to synchronise our body rhythms to the 24-hour day.

(2) Point: Key Research to support the role of exogenous zeitgebers in controlling our biological rhythms comes from Campbell and Murphy (1998). Example/Evidence: For example, Campbell and Murphy (1998) monitored the body temperatures of 15  volunteers who slept in a laboratory. They introduced light to the during the night at a series of intervals by shining a beam of light onto the back of their knees. They were woken at different times and a light pad was shone on the back of their knees. The participant’s circadian rhythms were disrupted by up to three hours. Elaboration:
This shows that it is not necessary for light just to enter the eyes to have a physiological effect on biological rhythms and shows the EZ do have an effect on our biological rhythms.

AO1: Biological Rhythms – Circadian Rhythms

Circadian Rhythm definition: a biological rhythm that occurs once every 24 hours. An example of this is the sleep/wake cycle.

Our circadian rhythms are driven by our body clocks found in all of the cells in our body and synchronised by the master circadian pacemaker the suprachiasmatic nucleus (SCN), found in the hypothalamus. This pacemaker must constantly be reset so that our bodies are in synchrony with the outside world. Light provides the primary input to this system, setting the body to the correct time.

Key Study: Circadian Rhythms:
Michel Siffre (1975):(he was a French cave explorer)
Aim: To establish the role of light as a zeitgeber in the sleep /wake cycle.
Michel Siffre in (1975), spent 6 months underground in a cave in Texas, in dim light; he had no cues as to the time of day. He had food, water, exercise equipment and books. He had a telephone link to the outside world which was staffed all the time. Siffre was wired up via computer and video so that his bodily functions could be monitored all of the time.
Findings: Siffre’s bodily rhythms were erratic at first (he went into a state of internal desynchronisation), but they soon settled down into fairly regular cycles of activity/inactivity e.g., sleep/wake, eating, etc. An interesting finding was that the daily cycle that he established seemed to run to 25 hours (approx’) and not 24 hours. However, sometimes his cycle would change dramatically and run over 48 hours. This meant that when he emerged from his period of isolation, he had “lost” a considerable number of days, it was actually the 179th day, whereas by his “days” it was only the 151st. He therefore LOST 28 days!
Conclusion: The pattern of sleep and waking remains even when there are no external cues. It appears that the natural length of the Circadian cycle is 25 hours.

This research tells us a number of things about EPs and EZs:

(1) Circadian rhythms are mainly controlled by EPs (specifically the SCN).   (2) naturally, we have a body clock that runs to 25 hours.                                   (3) Although our circadian rhythms are mainly controlled by EPs, the rhythm are modified by EZs to keep us in line with the 24 hour world.         (4) The EZ of light is important in keeping our clocks in line with the 24 hour world.

AO3: Evaluation of Research into Circadian Rhythms:


(1) Point: There is research to support the interaction between endogenous pacemakers and exogenous zeitgebers in the control of circadian rhythms.
Evidence/Example: For example,
• Aschoff and Wever (1976) placed participants in an underground WWII bunker with artificial light but no natural light and a complete lack of environmental and social cues. They found that most participants soon settled into a sleep/wake cycle of between 24 and 25 hours, although some rhythms were as long as 29 hours.
In addition,
• David Lafferty remained in a cave for 127 days (just over 4 months). Lafferty’s cycles appeared erratic at first, but then also began to settle at around 25 hours. Elaboration: This is a strength because, both pieces of research highlight the importance of internal factors (EPs) in the control of circadian rhythms. In addition, such research also highlights that EPs and EZs work together in order to keep our body in line with the 24 hour external world (i.e. without the EZ of light, our natural circadian sleep/wake rhythm would run to approx. 25 hours).


(1) Point: There are a number of problems with this research:
Lots of the research carried out into the control of circadian rhythms can be criticised for being case studies. Example/Evidence: For example, David Lafferty and Michel Siffre have carried out isolation studies (involving only themselves) in an attempt to investigate the control of circadian rhythms in relations to EPs and EZs. Elaboration: This is a weakness because, the evidence is based on only a select few individuals, this means that population validity is low and therefore there are difficulties when it comes to generalising the finings to the whole population.

(2) Point: In addition, much of the research looking at the control of circadian rhythms can be criticised as being androcentric. Evidence/Elaboration: For example, research from Siffre and David Lafferty are only looking at the control of circadian rhythms from a male perspective (i.e. they use male participants). Elaboration:  This is a weakness because the findings from this research only tells us about the EP and EZ control of male circadian rhythms. Females are physiologically different to males (e.g. they experience different biological processes such as the menstrual cycle) and therefore, it could be the case that female sleep/wake cycles are controlled in different ways by different internal and external processes.

AO1: Biological Rhythms – Ultradian Rhythms:

An Ultradian Rhythm is a rhythm that occurs more than once in a 24 hour period. An example of an Ultradian rhythm is the stages of sleep.

Discovering the rhythm of sleep: Hans Berger (1919) developed the electroencephalograph (EEG); this records electrical activity in the brain.

AO1: Research into Ultradian Rhythms:

Dement & Kleitman (1957) used the EEG to identify the systematic changes in brain waves that occur at regular intervals during sleep
• Using 9 male participants for up to 61 nights in a laboratory, they found 2 distinct kinds of sleep; REM sleep (when rapid eye movements occur) and NREM sleep (no rapid eye movements).

• They found that sleep is an active state that is made up of a number of different identifiable stages and that the average time spent in one Ultradian cycle (one complete NREM and REM cycle) was approx’ 90 minutes.

• When they woke participants during REM ~ 79% of the time (average) a dream was reported (7% reported dreaming when woken from NREM).

• They concluded that Ultradian cycles of sleep are made up of alternate cycles of NREM/ REM sleep, with the latter being associated with dreaming (but not exclusively so!)

Stages of Sleep Diagram outlining the 90 minute sleep cycle.

In a typical sleep laboratory, a volunteer settles down for the night with EEG

(electroencephalogram) wires attached (to measure brain activity), but also wires from an EOG (measuring eye activity) and from an EMG (measuring muscle activity).
Using these measurements, it was found that:
• A normal night’s sleep comprises a number of Ultradian cycles (approx’ 5).
• Each Ultradian sleep cycle lasts for approximately 90 minutes.
• Four stages of NREM and one stage called REM sleep has been identified.

Rechtschaffen and Kales (1968) developed criteria to describe changes in the brain’s electrical activity during sleep. These divide NREM sleep into four stages, each of which is characterised by distinct patterns of electrical activity.

Stages of Sleep Diagram

AO3: Evaluation of Ultradian Rhythm Research:


(1) Point: The research into ultradian rhythms can be praised for being conducted in controlled laboratory settings. Evidence/Example: For example, research looking at the stages of sleep uses EEG and ERP mechanisms (to name a few) to measure the waves occurring in the brain. Elaboration: This is a strength because, research from Dement and Kleitman and Rechtschaffen and Kales can be seen to produce scientific and objective measures that allow research to draw firm conclusions about the specific characteristics associated with the different stages of sleep (e.g. brain activity, muscle acidity, eye activity) which increased internal validity.


(1) Point: Due to the fact that research is conducted in the control setting of a laboratory, the sleep environment can be criticised for its artificial nature. Evidence/Example: For example, sleep research involves individuals sleeping under circumstances that don’t usually reflect their ‘typical’ nights sleep. Participants sleep with electrodes attached to their body/head, are often woken up a numbers of times throughout the night in order to report dreams etc… Furthermore, they are sleeping in a lab setting not their usual bedroom setting. Elaboration: This is a weakness because the research can be criticised for lacking ecological validity and not reflecting an individuals true sleep behaviour.

(2) Point: In addition, much of the research looking at the ultradian sleep cycle can be criticised as being androcentric. Example/Evidence: For example, Derment and Kleitman use an all male sample as does most early research investigating Ultradian rhythms. Elaboration: This is a weakness because the findings from this research only tells us about the sleep stages patterns of males. Females are physiologically different to males (e.g. they experience different biological processes such as the menstrual cycle) and therefore, it could be the case that female sleep patterns/cycles are very different (e.g. they could experience more or less slow wave/REM sleep in comparison to males).

AO1: Biological Rhythms – Infradian Rhythms

Infradian Rhythms are a biological rhythm that occurs less than once every 24 hours. An example of an Infradian rhythm is the female menstrual cycle occurring once every 28 days (approx.).

The Physiology of the Infradian cycle of Menstruation:
1. This relates to activity in the endocrine system that prepares the womb for the possibility of conception after egg cells are released.
2. Several hormones are involved and these are co-ordinated by the pituitary gland. Each cycle (which lasts for 28 days) ends with menstruation (unless pregnancy occurs).
3. The pituitary gland may be influenced by levels of light and the secretion of melatonin and this is supported by research.
This suggests that the infradian rhythm of menstruation is controlled mainly by internal factors (endogenous pacemakers).

AO1: McClintock – Dormitory effect – Menstrual Synchrony Studies
McClintock (1971) observed that the cycles among her dormitory friends at university became synchronised. After further research, she concluded that the cycles of 135 women (aged 17 – 22 years) became synchronised due to pheromones (“smell” chemicals that when secreted, cause specific reactions in other individuals).
McClintock and Stern (1988) began a follow-up 10-year longitudinal study that involved 29 women between the ages of 29 and 35 with a history of irregular, spontaneous ovulation. They obtained samples of pheromones from 9 of the women at certain points in their cycles by placing cotton wool pads under their arms – the women wore these pads for at least 8 hours! These pads were then treated with alcohol (to kill bacteria and disguise the smell) and then wiped under the noses (on the top lips) of the other 20 women on a daily basis.
Results 68% of the women responded to the pheromones. Menstrual cycles became synchronised to that of the pheromone donor.
Conclusion: The pheromones acted as an exogenous zeitgeber and modified the women’s Infradian rhythm of menstruation.

Key Study:

Aim: Infradian rhythms Reinberg (1967) aimed to investigate the influence of light on biological rhythms.
Procedure: Reinberg studied a young woman who spent 3 months in a cave relying only on the dim light from a miner’s lamp. There was no external light source. The effects on the sleep/wake cycle and the Infradian cycle of menstruation were noted.
Results: As in the Siffre study, the woman’s day lengthened to 24.6 hours. Her menstrual cycle shortened to 25.7 days. It took over 12 months before her menstrual cycle returned to normal.
Conclusions: The lack of light as a zeitgeber resulted in changes to both the circadian rhythm of the sleep/wake cycle and to the Infradian rhythm of menstruation, which was slow to adapt to the previous pattern even when light was restored.

AO3: Evaluation of Research into Infradian Rhythms:


(1) Point: Further research has supported the role of pheromones (EZs) along with EPs in the control of the infradian rhythm of menstruation. Evidence/Example: For example, Sabbagh and Barnard (1984) found that women who spend a lot of time together often find their menstrual periods become synchronised. It is suggested that this is due to the unconscious detection of chemical scents (pheromones) secreted at certain times during the menstrual cycle. Elaboration: This is a strength because it further supports the idea that infradian rhythms such as the menstrual cycle are controlled mainly by internal biological processes (EPs) but, can be modified by external factors (EZs, for example, pheromones).


(1) Point: Due to the fact that research into this area tampers with the infradian rhythm of menstruation, research can be criticised as being unethical. Evidence/Example: research from McClintock have purposely changed the length of its female participant menstrual cycles. Elaboration: This is a weakness because researchers are not fully aware of the implications of modifying/tampering with such cycles. Such changes could negatively affect female biological processes which is a lack of protection of participants.

(2) Point: In addition, much of the research is based on field experimental methods and therefore the is a lack of control over extraneous variables. Evidence/Example: For example, researchers such as McClintock are unable to control other external factors that could affect the female menstrual cycle (e.g. stress, exercise/physical activity etc…). Elaboration: This is a weakness because the lack of control over such variables makes it difficult for researchers to confirm ca cause and effect relationship between the presence of pheromones and the lengthening/shortening of the female menstrual cycle.