What Are Circadian Rhythms?

Core Concepts

Rhythms come in several time scales

Biological rhythms are classified by period. Circadian rhythms cycle about once per day (~24 h). Ultradian rhythms cycle more than once per day — examples include the ~90-minute sleep-cycle oscillation and the ~4-hour feeding rhythm in many mammals. Infradian rhythms cycle less than once per day — the human menstrual cycle (~28 days) and seasonal rhythms in many animals are examples. The circadian class gets the most attention because it is nearly universal and because a single master clock in the brain coordinates it. The terminology matters on exams: naming a rhythm correctly tells you its time scale and hints at the mechanisms that generate it.

The clock is endogenous

The key discovery about circadian rhythms is that they persist without external cues. In constant conditions — no light-dark cycle, no temperature change, no social cues — activity and physiology continue to cycle, but with a period that drifts away from 24 hours. This free-running period is the clock's intrinsic period, typically around 24.2 hours in humans in the classic laboratory studies (though estimates vary across studies and individuals). This is the strongest evidence that the rhythm is generated internally rather than simply driven by the sun. It also explains a practical quirk: a free-running period slightly longer than 24 hours means that, left to our own devices without cues, most people would drift later and later each day.

Entrainment and zeitgebers

Because the intrinsic period is not exactly 24 hours, the clock must be reset daily to stay synchronized with the external world. Entrainment is the process of synchronizing the internal rhythm to environmental cycles, and the cues that do the synchronizing are called zeitgebers (German for "time givers"). Light is the dominant zeitgeber for most organisms, but food availability, temperature, social cues, and exercise can also entrain rhythms — especially for peripheral clocks. The clock does not reset instantly; a phase response curve (PRC) describes how much and in which direction the clock shifts when light is presented at different points in the cycle. Light late in the subjective day pushes the clock later; light in the early subjective morning pushes it earlier. This phase-dependence is why jet lag recovery has a direction — eastward travel is generally harder than westward.

Why have an endogenous clock?

An internal clock is useful because it allows the body to anticipate regular changes rather than merely react to them. Hormones can rise before waking, body temperature can prepare for activity, and digestive enzymes can be ready before a meal arrives. Anticipation is more efficient than reaction: the system is prepared in advance instead of responding after the fact. The clock also lets the organism integrate many rhythms with each other — coordinating physiology across the day — and lets it maintain rhythmicity through brief environmental disturbances. This predictive, coordinating function is why the clock is conserved across nearly all life, from single-celled organisms to humans.

Measuring rhythms and the free-run experiment

The standard way to study a circadian rhythm is to place an organism in constant conditions and record a behavior (activity, sleep-wake, hormone level) over many days, displayed in an actogram — a chart with successive days stacked vertically. When the rhythm is entrained, activity onsets align vertically day after day. In constant darkness, a free-running animal's onsets drift progressively earlier or later, producing a diagonal pattern whose slope reveals the period. Such experiments showed that human free-running periods cluster near 24 hours and vary across individuals, and that bright light can shift the rhythm — paving the way for light-based treatments of circadian disruption discussed later in this chapter.

Common Confusions