Circadian Rhythm Calculator

1. Enter your work-day sleep. The time you usually fall asleep and wake up before work or school.
2. Enter your free-day sleep. The times on days off when you sleep as long as you like, ideally without an alarm.
3. Pick your work days per week. The calculator weights both kinds of night to find your weekly average sleep.

The chart redraws from your times above. It runs from noon to noon, with the night shaded, so your whole sleep fits on one line. On a phone, swipe the chart sideways to see the whole day.

• Work days and free days: your actual sleep. The sideways shift between the two bars is your social jetlag.
• Body clock: the sleep window your clock prefers for your usual amount of sleep, centred on your corrected mid-sleep.
• Light effect: bright light in the evening hours before that window tends to move your clock later, and daylight in the first hours after it tends to move it earlier. The shaded hours are a guide, not exact limits.
• Dashed line: the estimated middle of your melatonin night, about 4 hours before your free-day wake time.

1. Log a normal week. Note when you fall asleep and wake up, separately for work days and free days.
2. Let free days run naturally. Skip the alarm on at least two free days, or your free-day times will mirror your work days.
3. Enter the averages above. Your corrected mid-sleep is the single best number for your body clock.
4. Check the gap. Social jetlag over 2 hours means your schedule and clock are pulling apart.

To see how regular your timing is across the week, use the sleep consistency calculator.

A circadian rhythm is the roughly 24-hour cycle of changes in your body and behavior, driven by an internal clock. The master clock is a group of nerve cells in the brain called the suprachiasmatic nucleus. It uses the light reaching your eyes to set its time and passes that day and night signal on to clocks in the rest of the body.

Everyone has a circadian rhythm, but its timing differs. Your chronotype is that timing: early types are sleepy and alert earlier in the day, late types later. Chronotype is spread smoothly across the population, with most people somewhere in the middle.

Light is the strongest signal for the clock, which is why shift work and travel are so disruptive; see the shift work sleep calculator if your hours change.

It uses the arithmetic of the Munich ChronoType Questionnaire (MCTQ), a research method developed by Till Roenneberg and colleagues in Munich, which judges your body clock by the middle of your sleep on free days. This page is not the MCTQ and does not reproduce its questions: it applies the published formulas to the four sleep times you enter.

1. Free days: asleep 12:30 AM to 9:00 AM = 8 h 30 min, so mid-sleep is 4:45 AM.
2. Work days: asleep 11:30 PM to 6:30 AM = 7 h, so mid-sleep is 3:00 AM.
3. Weekly average with 5 work days: 7 h 26 min.
4. You sleep about 1 h 04 min longer than average on free days, so half of that is removed: mid-sleep becomes 4:13 AM.
5. Social jetlag: 4:45 AM – 3:00 AM = 1 h 45 min.

The correction matters because most people pay back work-week sleep debt by sleeping in, which makes them look later than their clock really is.

Every work week this person lives almost four time zones away from their own clock and sleeps 2.5 hours less on work nights than on free nights. Morning daylight and keeping weekend wake times close to work days are the usual first steps; very late timing that will not shift is worth discussing with a doctor.

The chronotype result comes from one number, your corrected mid-sleep. The table shows each band the calculator uses, how common it is, and the bedtime your clock would choose if you need 8 hours (the natural sleep window is your mid-sleep minus half your sleep).

Shares come from about 185,000 entries in the MCTQ database. The researchers note that the group borders are arbitrary: chronotype is a continuous scale, so a result near a border is not meaningfully different from the group next to it.

Your body clock switches on melatonin release in the evening. Researchers mark the moment with the dim light melatonin onset (DLMO), measured from saliva, blood or urine samples taken in dim light. It is the gold-standard marker of body-clock timing, and MCTQ mid-sleep has been checked against it.

• Onset: in young healthy adults sleeping at their usual times for a week, melatonin onset came about 2 hours before bedtime and about 14 hours after waking.
• Best predictors: in that study, wake time and mid-sleep tracked melatonin onset more closely than bedtime did, which is why this calculator works from mid-sleep.
• Middle of the melatonin night: about 4 hours before habitual wake time; the result above shows it as your deepest biological night.

So if your natural sleep window starts at 12:30 AM, your own melatonin is probably rising from roughly 10:30 PM. This is a rough population estimate, not a measurement. This section is about the melatonin your body makes; for melatonin supplements, talk to a doctor or pharmacist.

Light Timing

Light before the low point of your body clock pushes it later, and light after the low point pulls it earlier. In practice, bright morning light shifts you earlier and bright light late in the evening shifts you later.

Age

Teenagers have the latest body clocks; chronotype then moves earlier through adult life.

Genes

Part of the spread between people is inherited, which is why some people stay late types despite good habits.

Weak Daytime Light

Spending days indoors and evenings under bright screens gives the clock a weak signal, and it tends to drift later.

The two agree well in most people. Shift workers and people without regular free days get less reliable MCTQ results.

• People who are tired on work days but fine on weekends, a classic sign of social jetlag.
• Night owls trying to move their schedule earlier.
• Travellers who want to know their starting point; the jet lag recovery calculator plans the shift.
• Parents of teenagers, whose late clocks clash with early school starts.

Using Bedtime Instead Of Sleep Time

Enter when you fall asleep. Time spent reading or scrolling in bed is not sleep.

Using Alarm-Driven Free Days

If an alarm or a child wakes you on days off, your result will look earlier than your clock.

Judging From One Weekend

Average a few weeks. A late party or a trip will skew one weekend.

Fighting The Clock Overnight

Shifting a body clock takes days to weeks. Move bedtime gradually, by 15 to 30 minutes at a time.

The calculator follows the published MCTQ arithmetic, but your result is only as good as your sleep times and assumes a regular work and free-day pattern.

Very late or very early timing that you cannot shift may be a circadian rhythm sleep disorder, such as delayed sleep-wake phase. A sleep doctor can assess it.

It plots your body-clock timing across 24 hours. This one shows your work-day and free-day sleep, your natural sleep window, the middle of your melatonin night and when light shifts your clock.

Note when you fall asleep and wake up on free days without an alarm. The middle of that sleep, corrected for catch-up sleep, is the best simple marker of your body clock.

The most common group is intermediate, with a corrected mid-sleep between about 3:30 and 4:30 AM. About 30% of people fall in it.

Social jetlag is the gap between the middle of your sleep on work days and on free days. Over 2 hours is a large mismatch.

Partly. Morning daylight, less evening light and steady wake times can move a late clock earlier, but genes and age set the general range.

Late timing comes from genes, age and light. Teenagers are latest, and indoor days with bright evenings push the clock later.

For most people the middle of the melatonin night is a few hours before their natural wake time, roughly 4 hours before on average.

Not in the research sense. The four animal types come from a popular online quiz by Dr. Michael Breus, and figures such as bears making up about 55% of people are his estimates, not survey results. Researchers measure chronotype with sleep timing tools such as the MCTQ or with morningness questionnaires, and find that it runs on a continuous scale rather than falling into four fixed types. The dolphin type also describes insomnia-like sleep more than body-clock timing.

Sleeping in pays back sleep debt, but large shifts create social jetlag. Keeping free-day wake times within about an hour of work days is easier on the body clock.

See a doctor if you cannot fall asleep or wake at needed times for months despite good habits, especially if it affects work or school.

Only roughly. The method assumes regular work and free days, so rotating or night shifts give less reliable results.

• Roenneberg et al. Chronotype and social jetlag: a (self-) critical review (Biology, 2019) (MCTQ formulas, chronotype and social jetlag distributions)
• Khalsa et al. A phase response curve to single bright light pulses in human subjects (J Physiol, 2003) (light timing shifts; melatonin midpoint vs wake time)
• Burgess et al. The relationship between the dim light melatonin onset and sleep on a regular schedule in young healthy adults (Behav Sleep Med, 2003; PMID 15600132) (melatonin onset about 2 h before bedtime and 14 h after wake)
• Sleep Foundation: Chronotypes (origin of the lion, bear, wolf and dolphin types; chronotype as a spectrum)
• AASM Sleep Education: Delayed sleep-wake phase (when late timing becomes a disorder)