biological rhythms

Biological rhythms are cyclical patterns in behaviour and physiology regulated by internal biological clocks.

1. Circadian Rhythms (≈24 hours)

  • Example: sleep–wake cycle

  • Controlled by the suprachiasmatic nucleus (SCN) in the hypothalamus

  • Influenced by light (resets the body clock)

2. Infradian Rhythms (>24 hours)

  • Example: menstrual cycle (~28 days)

  • Regulated by hormones such as oestrogen and progesterone

3. Ultradian Rhythms (<24 hours)

  • Example: sleep stages (REM/NREM cycles)

  • Occur multiple times within a 24-hour period (≈90-minute cycles)

Endogenous Pacemakers & Exogenous Zeitgebers

  • Endogenous pacemakers: internal body clocks (e.g. SCN)

  • Exogenous zeitgebers: external cues (e.g. light, social cues) that reset rhythms

key strength is strong research support for the role of endogenous pacemakers. Studies such as Michel Siffre showed that when individuals are isolated from natural light, their sleep–wake cycle still follows a rhythm close to 24 hours.This suggests that biological rhythms are innate and internally controlled, supporting the role of the SCN. The use of controlled conditions increases the validity of the findings.

Research into biological rhythms has important real-world applications, particularly in understanding sleep disorders and shift work. For example, knowledge of circadian rhythms has led to treatments such as light therapy, which can reset disrupted body clocks.This increases the practical value of the research, as it can improve health and wellbeing in everyday life.

Another limitation is that the explanation may be reductionist, as it focuses heavily on biological mechanisms such as the SCN while ignoring the role of cognitive and social factors. For example, social schedules (e.g. work or school) can significantly influence sleep patterns.This means biological rhythms are likely influenced by an interaction of internal and external factors, reducing the explanatory completeness of purely biological accounts.