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Biological rhythms are cyclical patterns of physiological and psychological activity that occur over regular time periods. They are governed by internal biological clocks (endogenous pacemakers) and influenced by external environmental cues (exogenous zeitgebers). The body is not a static machine that runs at a constant rate; it is a finely tuned oscillator whose temperature, hormone levels, alertness, and even pain sensitivity rise and fall in predictable waves across the day, the month, and the year. Understanding biological rhythms is essential for explaining the sleep-wake cycle, the disruptive effects of shift work and jet lag, and the relationship between disturbed rhythms and mental and physical illness.
Key Definition: A biological rhythm is a cyclical change in the body's physiological processes or behaviour that recurs at regular intervals, governed by endogenous pacemakers and entrained (synchronised) by exogenous zeitgebers.
This lesson addresses the following points in AQA A-Level Psychology (7182), Paper 2, Section A (Biopsychology):
Assessment objectives engaged: AO1 (the three rhythm types and their defining features; named examples — the sleep-wake cycle, the menstrual cycle, seasonal affective disorder, the stages of sleep; the role of the SCN, pineal gland and melatonin; the supporting studies by Siffre, Aschoff & Wever, DeCoursey, Morgan, Ralph and Campbell & Murphy), and AO3 (evaluation — methodological problems of cave and free-running studies, the use of animal models, individual differences, the interactionist relationship between internal and external influences, and the implications of beta-bias in menstrual-cycle research). The effect of pacemakers and zeitgebers on the sleep-wake cycle is a named requirement.
There are three main categories of biological rhythm, classified by their cycle length:
| Type | Cycle Length | Frequency | Examples |
|---|---|---|---|
| Circadian | Approximately 24 hours | Once per day | Sleep-wake cycle, core body temperature, cortisol secretion |
| Infradian | More than 24 hours (days, weeks, months) | Less than once per day | Menstrual cycle (~28 days), seasonal affective disorder (annual) |
| Ultradian | Less than 24 hours | More than once per day | Stages of sleep (~90-minute cycle), basic rest-activity cycle (BRAC) |
A useful memory aid is the Latin: circa = "about", dies = "day", so circa-dian means "about a day"; infra = "below", so an infradian rhythm has a frequency below one per day (it takes longer than a day to complete); ultra = "beyond/more", so an ultradian rhythm has a frequency of more than one per day.
graph TD
A[Biological rhythms] --> B[Circadian<br/>~24 hours]
A --> C[Infradian<br/>>24 hours]
A --> D[Ultradian<br/><24 hours]
B --> B1[Sleep-wake cycle]
B --> B2[Core body temperature]
C --> C1[Menstrual cycle ~28 days]
C --> C2[SAD ~12 months]
D --> D1[Stages of sleep ~90 min]
D --> D2[Basic rest-activity cycle]
The most studied circadian rhythm is the sleep-wake cycle — the 24-hour pattern of sleeping and waking. This cycle is regulated by an internal biological clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus, but it is also shaped by external cues, above all light.
Key Definition: A circadian rhythm is a biological rhythm that occurs approximately once every 24 hours, such as the sleep-wake cycle or daily fluctuations in body temperature and hormone levels.
Another important circadian rhythm is core body temperature, which is lowest at around 04:30 (roughly 36 °C) and peaks in the early evening (around 38 °C). The dip in temperature is associated with the deepest sleep and with the well-known mid-afternoon "post-lunch" trough in alertness. Cortisol, the hormone that promotes wakefulness, also follows a circadian pattern, peaking shortly after waking (the cortisol awakening response) and reaching its lowest point around midnight.
The SCN is a tiny cluster of around 20,000 neurons located in the hypothalamus, directly above the optic chiasm (where the optic nerves from each eye cross). Its position is no accident: it sits at the crossing of the visual pathways so that it can receive direct input from the retina about ambient light levels, even when we are not consciously seeing.
How the SCN regulates the sleep-wake cycle:
Key Definition: Melatonin is a hormone produced by the pineal gland that promotes sleep. Its secretion follows a circadian pattern — increasing in darkness and decreasing in light.
Michel Siffre, a French geologist, conducted case studies on himself by spending extended periods in underground caves with no cues to the time of day (no clocks, no natural light, no social contact).
Conclusions:
Aschoff and Wever (1976) placed participants in a deep underground WWII bunker, deprived of natural light and time cues. The majority of participants settled into a free-running circadian rhythm of slightly more than 24 hours (around 25 hours), although one participant's rhythm extended to as long as 29 hours. This corroborated Siffre's central finding using a larger sample: the natural human clock runs a little slow and must be reset daily.
A criticism of the earlier studies is that participants had access to artificial light (Siffre used a torch). Czeisler and colleagues (1999) controlled light far more tightly and found that the endogenous circadian cycle is actually much closer to 24 hours and 11 minutes, not the 25 hours suggested by the cave studies. They also showed that even dim artificial light can entrain the clock — which explains why the earlier estimates were too long.
Exam Tip: Use Czeisler et al. (1999) to update the cave-study estimates. The examiner-pleasing point is that the free-running cycle is close to — but slightly longer than — 24 hours, and that uncontrolled artificial light inflated the earlier figures. This is a neat way to evaluate Siffre while adding accurate detail.
An important qualification to all of the above is that circadian rhythms are not identical across people. Czeisler et al. found that free-running cycle length, although averaging close to 24 hours, varied between individuals. People also differ in their chronotype — their natural preference for morningness or eveningness. "Larks" (morning types) wake and peak earlier in the day, whereas "owls" (evening types) function best later and struggle with early mornings; Duffy et al. (2001) reported reliable differences in the phase of the circadian temperature rhythm between morning and evening types. These differences appear to be partly innate: variants of clock genes such as PER3 are associated with chronotype, with one version more common in morning types. The practical implication is that generalising about "the" sleep-wake cycle is misleading, and that schedules (school start times, shift allocation) ideally take chronotype into account.
Infradian rhythms take longer than 24 hours to complete one cycle.
The human menstrual cycle is an infradian rhythm averaging about 28 days (though normal cycles range from roughly 24 to 35 days), governed by the interaction of hormones: oestrogen, progesterone, follicle-stimulating hormone (FSH) and luteinising hormone (LH).
| Phase | Approx. days | Key events |
|---|---|---|
| Menstruation | 1–5 | Uterine lining sheds |
| Follicular phase | 1–13 | FSH stimulates follicle development; oestrogen rises |
| Ovulation | ~14 | A surge in LH triggers release of the egg |
| Luteal phase | 15–28 | Progesterone maintains the uterine lining; if no fertilisation occurs, hormone levels fall and menstruation begins |
The menstrual cycle is primarily controlled by endogenous hormonal feedback, but research has asked whether it can also be influenced by exogenous cues — specifically the pheromones of other women.
McClintock (1971) reported menstrual synchrony: she observed that women living in close proximity (in a US college dormitory) tended, over several months, to have menstrual onsets that drew closer together, suggesting an external chemosignal (pheromone) might act as a zeitgeber. A later study by Stern and McClintock (1998) exposed women to cotton pads carrying the underarm secretions of other women at different phases of their cycle and reported shifts in cycle length, supporting the pheromone hypothesis.
However, synchrony findings have proved difficult to replicate. Critics (e.g. Yang and Schank, 2006) argue that with cycles of variable length, some convergence and divergence of onset dates is expected by chance, and that confounding variables (diet, stress, exercise) are hard to control. Menstrual synchrony is therefore now regarded as, at best, a weak and unreliable effect.
SAD is a depressive disorder that follows a seasonal (annual) pattern — symptoms typically begin in autumn/winter and lift in spring/summer. Because the cycle spans roughly a year, it is classified as an infradian rhythm (sometimes specifically a circannual rhythm).
SAD is theoretically interesting because it sits at the intersection of the rhythm categories: the trigger is a circannual (infradian) change in day length across the year, but the proposed mechanism is a disruption of the circadian melatonin/serotonin system. It thus shows how the three categories of rhythm are not wholly separate but can interact — a point worth making in an extended answer.
The menstrual cycle, too, has been linked to mood: the luteal-phase fall in oestrogen and progesterone is associated by some researchers with premenstrual changes in mood, illustrating how an infradian hormonal rhythm can have psychological as well as physiological effects. As with synchrony, however, the evidence is mixed and the topic is sensitive, so claims should be made cautiously and without reinforcing stereotypes — a legitimate gender-bias point for AO3.
Ultradian rhythms repeat more than once within a 24-hour period.
The most important ultradian rhythm is the sleep cycle, which lasts approximately 90 minutes and repeats four or five times across a night. Each cycle moves through a sequence of NREM (non-rapid-eye-movement) stages followed by a period of REM (rapid-eye-movement) sleep, and the brain's electrical activity, monitored by EEG (electroencephalogram), changes characteristically at each stage.
Key Definition: An ultradian rhythm is a biological rhythm that occurs more than once in a 24-hour period, such as the stages of sleep.
| Stage | Type | EEG pattern | Key features |
|---|---|---|---|
| Stage 1 | NREM (light sleep) | Alpha → theta waves | Easily woken; hypnagogic experiences; few minutes |
| Stage 2 | NREM | Theta waves with sleep spindles and K-complexes | Harder to wake; heart rate and temperature fall |
| Stages 3 & 4 | NREM (slow-wave / deep sleep) | Delta waves (high amplitude, low frequency) | Very hard to wake; growth hormone released; physical restoration; sleepwalking/night terrors may occur |
| REM | REM | Beta-like waves (resemble waking) | Eyes dart; vivid dreaming; muscle paralysis (atonia); brain highly active — "paradoxical sleep" |
As the night progresses the balance shifts: early cycles are dominated by deep slow-wave sleep, whereas later cycles contain progressively longer periods of REM. This is why the dreams we remember on waking tend to come from the last sleep cycle of the night.
Dement and Kleitman (1957) conducted a landmark study using EEG to map the stages of sleep and the REM-dreaming link. Participants woken from REM sleep reported a dream around 80% of the time, compared with only about 7% from NREM sleep; reported dream length also matched the actual time spent in REM. They confirmed the roughly 90-minute ultradian cycle.
The proportions of each sleep stage also change predictably across the lifespan, which is further evidence that the sleep cycle is biologically programmed. Newborns spend around half of their sleep in REM and sleep in short, frequent bouts; the proportion of REM falls steeply through childhood, and in older adulthood slow-wave (deep) sleep declines markedly, which is one reason sleep becomes lighter and more fragmented with age. The fact that the architecture of sleep follows such a consistent developmental trajectory across individuals and cultures supports the idea of an endogenous ultradian programme rather than a purely learned pattern.
Kleitman (1969) proposed that the same ~90-minute rhythm continues during waking hours as the basic rest-activity cycle (BRAC): across the day, humans move from a period of alertness to a trough of fatigue and reduced concentration roughly every 90 minutes. This suggests the ~90-minute oscillator is not confined to sleep but is a fundamental ultradian rhythm of the brain. Evidence for the BRAC is more mixed than for the sleep cycle itself — waking activity is heavily overlaid by social demands and willpower, which makes a clean 90-minute waking rhythm hard to detect — so it remains a more tentative claim than the well-established nocturnal sleep cycle.
Exam Tip: Learn the EEG wave types in order of arousal: beta (alert waking), alpha (relaxed/drowsy), theta (light sleep), delta (deep sleep), with REM paradoxically resembling waking beta activity. Examiners frequently ask you to describe how brain activity changes across the sleep stages.
Endogenous pacemakers are internal biological clocks that generate and maintain biological rhythms even without external cues. The master pacemaker for circadian rhythms is the SCN, supported by the pineal gland and its secretion of melatonin.
Animal research has been pivotal in establishing the role of the SCN:
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