Biological Rhythms

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). Understanding biological rhythms is essential for explaining the sleep-wake cycle, the effects of shift work and jet lag, and the relationship between disrupted rhythms and mental health.

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 by exogenous zeitgebers.

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Types of Biological Rhythm

There are three main categories of biological rhythm, classified by their cycle length:

Type	Cycle Length	Examples
Circadian	Approximately 24 hours	Sleep-wake cycle, body temperature, cortisol secretion
Infradian	More than 24 hours (days, weeks, months)	Menstrual cycle (~28 days), seasonal affective disorder (annual)
Ultradian	Less than 24 hours	Sleep stages (~90-minute cycles), basic rest-activity cycle (BRAC)

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Circadian Rhythms

The Sleep-Wake 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.

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.

The Suprachiasmatic Nucleus (SCN)

The SCN is a tiny cluster of approximately 20,000 neurons located in the hypothalamus, directly above the optic chiasm (where the optic nerves from each eye cross). Its position is critical because it receives direct input from the retina about ambient light levels.

How the SCN regulates the sleep-wake cycle:

1. During daylight, light receptors in the retina (specifically, intrinsically photosensitive retinal ganglion cells containing the photopigment melanopsin) detect light and send signals to the SCN.
2. The SCN sends signals to the pineal gland, instructing it to suppress the production of melatonin.
3. In darkness, the SCN signals the pineal gland to increase melatonin production.
4. Rising melatonin levels promote drowsiness and sleep onset.
5. In the morning, light exposure suppresses melatonin, promoting wakefulness.

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's Cave Studies (1962, 1972)

Michel Siffre, a French geologist, conducted case studies on himself by spending extended periods in underground caves without any cues to the time of day (no clocks, natural light, or social contact).

• In 1962, he spent two months in a cave in the Alps. His sleep-wake cycle settled into a rhythm of approximately 24.5 hours — slightly longer than 24 hours.
• In 1972, he spent six months in a cave in Texas. His cycle again extended to approximately 25 hours, though it became more erratic over time (sometimes stretching to 48-hour cycles as the study progressed).

Conclusions:

• The existence of a regular sleep-wake cycle in the absence of external cues confirms the role of an endogenous pacemaker (internal clock).
• The cycle was not exactly 24 hours, suggesting that exogenous zeitgebers (especially light) normally entrain (reset) the internal clock to a precise 24-hour cycle.

Evaluation (AO3):

• Strength — demonstrates the existence of an endogenous clock, as the rhythm persisted without external cues.
• Limitation — case study of one person — cannot generalise to all individuals. Siffre's age, personality, and motivation may have affected results.
• Limitation — although light was controlled, Siffre had access to artificial light (e.g., torches), which may have partially acted as a zeitgeber.
• Limitation — living underground for months is highly unusual and stressful, which may have disrupted his natural rhythms.

Czeisler et al. (1999)

Charles Czeisler and colleagues (1999) conducted a more controlled study. They placed participants in an environment with no natural light and systematically varied the intensity of artificial light. They found that the endogenous circadian cycle was closer to 24 hours and 11 minutes (not 25 hours as previously thought), and that very dim artificial light could entrain the cycle.

Exam Tip: Cite Czeisler et al. (1999) to update the earlier estimates from Siffre. Examiners reward the use of more recent, controlled evidence. The key point is that the free-running cycle is close to — but not exactly — 24 hours.

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Infradian Rhythms

Infradian rhythms last longer than 24 hours.

The Menstrual Cycle

The human menstrual cycle is an infradian rhythm lasting approximately 28 days, governed by the interaction of hormones: oestrogen, progesterone, follicle-stimulating hormone (FSH), and luteinising hormone (LH).

Phase	Duration	Key Events
Menstruation	Days 1–5	Uterine lining sheds
Follicular phase	Days 1–13	FSH stimulates follicle development; oestrogen rises
Ovulation	Day 14	LH surge triggers release of the egg
Luteal phase	Days 15–28	Progesterone maintains the uterine lining; if no fertilisation, hormone levels drop and menstruation begins

Research on menstrual synchrony: McClintock (1971) reported that women living in close proximity (e.g., in college dormitories) tended to synchronise their menstrual cycles over time, possibly mediated by pheromones. However, subsequent research has failed to consistently replicate this effect (Yang & Schank, 2006), and the phenomenon is now considered unreliable.

Seasonal Affective Disorder (SAD)