Circadian rhythm calculator
Your body clock is set by when light hits your eyes — morning light pulls it earlier, evening light pushes it later. Enter your wake time, bedtime and chronotype and this calculator lays out the day's anchors: when to seek light, when to dim it, where caffeine and exercise fit, and where your alertness peak and circadian nadir fall.
Take the quiz if you don't know — bear timing is used for caffeine and exercise.
With a fixed 7:00 am wake and 11:00 pm bedtime, your clock's low point (the nadir) sits around 4:00 am–5:00 am — the hardest hours to be awake — and alertness peaks roughly 9:30 am–12:00 pm. Hold the wake time steady and get light early: those two do most of the work.
- 7:30 am
Bright light, 10+ minutesseek light
Light in the first hour after waking is the strongest signal your clock receives — it anchors the rhythm and pulls tomorrow's wake time earlier. Outdoors beats a window; a bright lamp works when the sky doesn't.
- 11:00 am
Second light dose around middayseek light
A second bright-light window around midday reinforces the anchor and supports the afternoon alertness signal.
- 2:00 pm
Last caffeinecaffeine
Caffeine's half-life is ~5–6 hours — a 2:00 pm cutoff leaves most of it cleared before 11:00 pm.
- 5:00 pm
Exercise window opensexercise
Afternoon-to-early-evening exercise raises core temperature and, as it falls afterwards, deepens the night's slow-wave sleep. Hard training within ~2 h of bedtime can delay sleep onset.
- 8:00 pm
Last substantial mealmeal
Finishing eating ~3 h before bed lets digestion settle — a full stomach raises core temperature and fragments early sleep.
- 9:00 pm
Dim the lightsavoid light
Bright evening light suppresses melatonin and delays the clock. Dimming overheads 2 h before bed lets the evening signal rise on schedule.
- 10:00 pm
Screens off or night-shifted and dimmedavoid light
Close-range bright screens in the last hour are the most common circadian delay in modern schedules. If you must use them, lowest brightness and a warm filter.
- 11:00 pm
Lights outsleep
A fixed 11:00 pm bedtime, held within ~30 minutes even at weekends, is what keeps the whole schedule anchored.
Circadian nadir
4:00 am – 5:00 am
Your clock's low point — the worst hours to be awake and the best proof your schedule is working if you sleep through them.
Alertness peak
9:30 am – 12:00 pm
Schedule demanding cognitive work here; it rides the rising phase of your core-temperature rhythm.
Planning tool, not medical advice. These figures are population averages for scheduling your sleep — they don't diagnose or treat any condition. See a clinician for persistent sleep problems.
How does the circadian clock work?
The suprachiasmatic nucleus runs slightly longer than 24 hours — about 24.2 in most people — so every day it must be reset, and the reset signal is light. Light before your temperature nadir (roughly the last third of sleep) advances the clock; light after it delays it. That phase-response curve is the entire mechanism behind jet lag, shift-work disorder, and every "fix my sleep schedule" plan that has ever worked.
Which levers move the clock most?
First: a fixed wake time, seven days a week — the single strongest stabiliser, because it fixes when the light anchor lands. Second: bright light within an hour of waking, ten minutes outdoors beating any indoor substitute. Third: dimming and screen discipline in the last two hours — the wind-down routine planner schedules exactly those hours backwards from your bedtime. Fourth, the supporting cast — caffeine timing, afternoon exercise, a cool room. Most people who think they have a sleep problem have a light-timing problem, and levers one and two fix the majority of them within two weeks.
Why does your chronotype change the schedule?
A lion and a wolf with the same wake time owe their clocks different debts: the wolf's biology is actively delaying every day, so evening light discipline matters more and the caffeine cutoff sits later. The calculator adjusts the caffeine and exercise anchors to your chronotype— take the quiz first if you haven't, then come back and regenerate the schedule.
FAQ
What sets the circadian rhythm?
Light — specifically, bright light hitting the retina's melanopsin cells, which wire directly to the suprachiasmatic nucleus, the brain's master clock. Light in the morning advances the clock (earlier timing); light in the evening delays it (later timing). Everything else — meals, exercise, temperature — is a secondary zeitgeber. Get the light timing right and the rest follows.
How much morning light do I need?
Ten minutes of outdoor light within an hour of waking is the practical target on a bright day; a grey winter morning can need 20–30. Indoors through a window is far weaker — typical room light is 100–500 lux against 10,000+ outside on an overcast day. If you cannot get outside, a 10,000-lux lamp at arm's length is the studied substitute.
Does blue-light blocking actually work?
The evidence is mixed and the effect modest compared to the obvious intervention: brightness and engagement. Dimming everything two hours before bed and putting stimulating screens away does more than any filter, because cognitive arousal suppresses sleep onset independently of light. Filters are a fine extra; they are not the intervention.
How do I fix a broken sleep schedule?
Anchor one thing and let the rest follow: a fixed wake time, held within 30 minutes seven days a week, with bright light immediately after it. The clock entrains to the wake anchor within one to two weeks. Trying to fix bedtime directly rarely works — you cannot command sleep onset, but you can command the alarm and the light that follows it.
How we source this
The sleep-duration figures come from the National Sleep Foundation's 2015 consensus panel (Hirshkowitz et al., Sleep Health) — 18 experts convened from 12 stakeholder organisations, reviewing 300+ studies. The cycle, REM, jet-lag and wind-down models are built from the primary literature, and every source is listed with its context on the methodology & sources page.
Content is written and maintained by the SleepMaxx Calculator editorial team and checked against those sources. This site is not medically reviewed— no clinician reviews this content, and we would rather say that in plain text than hang a reviewer byline we don't have. These are planning tools built on population averages; for a personal sleep problem, a clinician is the right next step.
You will also not find an Epworth Sleepiness Scale, a STOP-BANG questionnaire or an AHI estimator here. Those are clinical screening instruments: their scores only mean something in the hands of someone who can act on them, and hosting them on a calculator site is a safety problem, not a feature. We exclude them deliberately.
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