Definition
The circadian rhythm is the roughly 24-hour internal cycle that governs sleep, alertness, body temperature, hormone release, digestion, and mood across the day. It is generated by a master pacemaker in the suprachiasmatic nucleus of the hypothalamus, with peripheral clocks in nearly every tissue. The system is endogenous — it runs even in the absence of external cues — but is synchronised, or entrained, to the solar day by light, meal timing, and social activity.
Disruption of the circadian system is implicated in a long list of problems: shift-work disorder, jet lag, insomnia, seasonal affective disorder, and increased risk of metabolic and cardiovascular disease. The system is also of central interest to psychology because cognitive performance, mood, and emotional regulation all show pronounced circadian variation.
Why it matters
How it works
The suprachiasmatic nucleus generates the master rhythm through a feedback loop of clock genes whose protein products rise and fall over a roughly 24-hour cycle. Light hitting specialised retinal cells signals the time of day to this pacemaker and adjusts its phase. The pacemaker in turn drives the secretion of melatonin (rising in the evening, falling at dawn) and modulates the activity of peripheral clocks throughout the body.
Behaviourally, the system produces predictable cycles: alertness peaks in the late morning and early evening, with a midday dip; body temperature is lowest in the early hours of the morning; reaction time and working memory follow alertness. Misalignment — sleeping at hours the clock expects you to be active, or working under bright light at night — desynchronises the central and peripheral clocks and produces the symptoms of shift-work disorder. Bright light in the morning and dim light in the evening are the most effective behavioural interventions for keeping the system in tune.