Learn Chronobiology, from minutes to seasons

Classic three-cycle biorhythms vs your body’s biological rhythms

Your body does not keep just one clock

Sleep stages, temperature, hormones and seasonal responses all change over time. Scientists group these biological rhythms by how long one complete cycle takes.

Ultradian less than a day Circadian about a day Infradian longer than a day Circannual about a year
What is a rhythm?

A rhythm is a repeating change measured through time

The scientific field that studies biological timing is called chronobiology. Researchers observe a process, measure how it rises and falls, and test whether the pattern persists or responds to time cues.

Some rhythms come from internal biological clocks. Others are direct responses to the environment. The difference matters: something that happens every day is not automatically controlled by a circadian clock.

Heartbeat and breathing are fast biological oscillations, but they are not usually what scientists mean by an ultradian clock. Ultradian examples more directly include recurring sleep stages and pulses of hormone release.

The rhythm spectrum

Four families, separated by time

The boundaries are useful labels rather than walls. Different textbooks use slightly different cut-offs, but the central idea remains the same.

01

Ultra diem, more than once a day

Ultradian rhythms

A sleeping person beneath several repeating dream and sleep waves
Several cycles can unfold during one night.

These repeat within a 24-hour day. During sleep, humans normally cycle through non-REM and REM sleep every 80 to 100 minutes, usually four to six times per night.

  • REM and non-REM sleep cycling
  • Pulses of hormone secretion
  • Recurring feeding or activity patterns
See the full sleep-cycle guide
Where the word comes from

It comes from the Latin:

  • ultra: beyond
  • diem: day

Ultradian describes a rhythm occurring more than once within a day, so each complete cycle lasts less than 24 hours.

02

Circa diem, about a day

Circadian rhythms

One person shown through a continuous transition from sunrise and daytime activity to night and sleep
One internal day, continually aligned to the world outside.

Light is the strongest time cue, helping this internal cycle stay aligned with the day outside.

  • Sleep and wake timing
  • Core body temperature
  • Melatonin and cortisol timing
See why time zones cause jet lag
Where the word comes from

It comes from the Latin:

  • circa: approximately or around
  • diem: day

A circadian rhythm is an internally generated biological cycle lasting roughly 24 hours. Examples include the sleep-wake cycle, body temperature and the daily timing of hormones such as melatonin.

03

Infra diem, less than once a day

Infradian rhythms

A woman beside a circular progression of buds, flowers and resting leaves representing change across several weeks
A longer rhythm can develop gradually across weeks.

These take longer than 24 hours to complete. The menstrual cycle is a familiar human example, but it is not a precise 28-day clock and varies between people and across months.

  • Menstrual-cycle physiology
  • Some immune and hormonal patterns
  • Cycles extending over days or months
Read the NICHD cycle guide
Where the word comes from

It comes from the Latin:

  • infra: below or beneath
  • diem: day

Infradian means the rhythm occurs less than once per day, so one complete cycle lasts longer than 24 hours.

04

Circa annum, about a year

Circannual rhythms

One tree and migratory birds shown through spring, summer, autumn and winter
A yearly rhythm follows the changing seasons.

These follow an approximately yearly pattern and sit within the broader infradian family. They are especially clear in seasonal reproduction, migration and hibernation in animals.

  • Animal migration and hibernation
  • Seasonal reproduction
  • Season-linked human changes
Understand seasonal affective disorder
Where the word comes from

It comes from the Latin:

  • circa: approximately or around
  • annum: year

Circannual therefore means about a year and describes biological patterns that recur approximately once each year.

Four remarkable clock stories

Biological time turns up in unexpected places

These discoveries show why chronobiology is much more than a sleep topic.

1729

A plant kept time in darkness

Astronomer Jean Jacques d'Ortous de Mairan placed a mimosa plant in darkness. Its leaves continued opening and closing at the expected times, an early clue that daily timing can arise inside an organism.

Read the Nobel background
2017

The body clock won a Nobel Prize

Jeffrey Hall, Michael Rosbash and Michael Young received the Nobel Prize for discoveries showing how clock genes and their proteins build a self-regulating molecular loop.

Explore the molecular clock
16

Astronauts see 16 sunrises a day

The International Space Station circles Earth so quickly that natural sunrise is no longer a useful daily clock. Programmable lighting helps simulate an Earth-like light and dark cycle.

See NASA's sleep research

Medicine can have a timing question

Chronotherapy studies whether aligning a treatment with biological timing can improve its effect or reduce unwanted effects. Timing is treatment-specific, so medication schedules should only be changed with appropriate clinical advice.

See the medical definition
One conductor, many players

Your brain coordinates clocks throughout the body

The suprachiasmatic nucleus, or SCN, is a small region in the hypothalamus that receives information about light from the eyes. It acts as a master coordinator for circadian rhythms.

Nearly every tissue and organ also has its own molecular clock. The SCN helps these clocks stay aligned with one another and with the outside day.

Explore the NIH circadian guide

Light is the strongest cue

Morning and evening light help move the phase of the circadian system and align it with local time.

Food gives timing information

Meal timing can influence clocks in organs involved in digestion and metabolism.

Activity is another signal

Movement, social routines, stress and temperature can also affect biological timing.

Clocks can disagree

Jet lag and shift work move behaviour faster than the body can fully adjust.

Beyond the 24-hour day

Moon, tides and seasons

Nature provides many repeating cues, but similar lengths do not prove the same cause. A roughly monthly human process is not automatically controlled by the Moon.

Circatidal

Life around the tides

Many coastal organisms time feeding, movement or reproduction around the approximately 12.4-hour tidal cycle. These are powerful examples of biology tuned to a repeating environment.

Circalunar

Life around the lunar month

Some marine species synchronise behaviours with moonlight or lunar-linked tides. Evidence for strong, general lunar control of human physiology is far less certain.

Explore Moon phases
Seasonal

Life around the year

Changing day length helps organise migration, reproduction, coat changes and hibernation. Human mood and sleep can also show seasonal influences.

Two ways of seeing rhythm

Classic curves begin with simple, memorable rules

One tradition calculates repeating curves from a birth date; the other observes repeating changes in the body. Together they show how deeply people are drawn to order, timing and cycles.

Classic biorhythm curvesBiological body rhythms
Foundation

Elegant mathematical curves calculated from a birth date.

Body clocks involving genes, cells, nerves, hormones and environmental cues.

Timing

Follows the classic 23, 28 and 33-day periods throughout life.

Can shift with light, food, activity, age, health and environment.

How it is read

Physical, emotional and intellectual lines rise, cross and fall in a repeating personal pattern.

Sleep studies, temperature, hormones, gene activity and behaviour reveal changes over time.

Human meaning

People compare the curves with lived experience, intuition and the order they sense in the universe.

Chronobiology helps explain familiar experiences such as sleep timing, jet lag and seasonal change.

Read the history of biorhythms How our calculator works
The language of clocks

Six words that make chronobiology easier

01

Period

The time needed for one full cycle.

02

Frequency

How many cycles occur within a chosen time.

03

Phase

Where the rhythm currently sits in its cycle.

04

Amplitude

How far the signal rises and falls.

05

Entrainment

Alignment of an internal rhythm to an outside cue.

06

Zeitgeber

A time-giver such as light that helps set a clock.

Questions people often ask

Biological rhythms FAQ

No. The NIH describes cycles restarting every 80 to 100 minutes, and cycle length and composition change across the night. Ninety minutes is a convenient average, not an alarm-clock rule.

Not quite. Diurnal describes a pattern associated with daytime and night-time. Circadian usually refers to an internally generated, approximately 24-hour rhythm that can persist without external time cues and can be entrained by light.

The average is often described as 28 days, but healthy cycles vary between people and from month to month. Similarity to the lunar month does not by itself prove that moonlight or lunar phase controls the human menstrual cycle.

They come from the classic mathematical biorhythm tradition, while chronobiology observes rhythms directly in the body. The two approaches share a fascination with cycles, but explore them in different ways. Many people enjoy the classic curves symbolically, much like numerology, horoscopes, feng shui or tarot cards, and compare them with their own experiences.