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.

What this dot point is asking

AQA wants you to describe circadian, infradian and ultradian rhythms and explain the role of endogenous pacemakers and exogenous zeitgebers. The exam skill is to define the three rhythms by duration with correct examples, and to explain how internal clocks and external cues interact to control the sleep/wake cycle.

The three rhythms

The three rhythms are distinguished purely by how long one full cycle takes. The circadian sleep/wake cycle is the headline example, governing alertness and the daily release of hormones such as cortisol and melatonin. Infradian rhythms run on a longer-than-daily cycle: the menstrual cycle (about 28 days) is regulated by hormones, and there is evidence that it can be influenced by external factors, since McClintock found that pheromones could synchronise the cycles of women living together. Seasonal affective disorder (SAD) is treated as a yearly infradian rhythm, in which longer winter nights raise melatonin and lower mood. Ultradian rhythms run on a shorter-than-daily cycle: the stages of sleep recur roughly every 90 minutes, moving through light sleep (stages 1 and 2), deep slow-wave sleep (stages 3 and 4) and rapid eye movement (REM) sleep associated with dreaming, identifiable through EEG patterns. Being able to tag each rhythm with its duration and a hormonal or EEG example is what distinguishes a top answer.

Pacemakers and zeitgebers

The sleep/wake cycle is controlled by an interaction between internal and external factors. The endogenous pacemaker is the suprachiasmatic nucleus (SCN), a tiny region of the hypothalamus that acts as the master clock. It generates its own roughly 24-hour rhythm and signals the pineal gland to release melatonin at night, promoting sleep. The SCN does not work in isolation: exogenous zeitgebers (German for "time-givers"), above all light, reset or entrain the clock each day so it stays locked to the 24-hour environment. Light is detected partly through the eyes and fed to the SCN, which is why jet lag and shift work, which disrupt the normal light cycle, throw the rhythm out. The classic evidence is Siffre's cave studies: living for months without natural light or clocks, his free-running cycle drifted to about 24 to 25 hours, showing the SCN maintains a near-daily rhythm on its own but needs external cues to stay precisely synchronised. This interactionist conclusion is the strongest evaluative line.