Body Insights' Circadian Timing Estimate shows your peak alertness range. It also gives you times for light, exercise, caffeine, and bedtime. The Circadian Rhythm card plots these guides on a daily alertness curve. It also marks estimated Melatonin Onset. The estimate reflects your circadian phase, or where your body clock sits in its daily cycle. Your Apple Watch records sleep and heart rate. Apple Health stores those records. Body Insights builds the curve from your latest usable sleep period. It also uses overnight heart rate when available. The sleep record must have enough data to support the estimate.
You can find the card in the Insights tab of Body Battery.
What does the Circadian Rhythm card show?
The card starts with your Peak alertness range, wake time, and bedtime. Open it to see the full alertness curve and five timing guides:
- Morning Light shows the scheduled light window for today.
- Peak Cognitive Window marks time for demanding mental work.
- Exercise marks a window for physical training.
- Caffeine Cutoff shows when to stop caffeine before bedtime.
- Melatonin Onset marks the estimated start of your evening melatonin rise.
The card rounds estimated times to the nearest half hour. It also has an I just ate button that can update the alertness curve after a meal.
Why does circadian timing matter?
The times on today's card can move when your sleep start, wake time, or light plan changes. A meal response can also update the alertness curve. Yesterday's times may no longer fit. Check the latest estimate before you plan focused work, exercise, caffeine, or bedtime.
Use the times as planning windows. If your symptoms or energy change, adjust the plan.
When to use circadian timing
- Save a demanding task for your Peak Cognitive Window when you can.
- Before a workout, check the Exercise window.
- Thinking about an afternoon coffee? See whether you have passed your Caffeine Cutoff.
- If your sleep shifts later, check Morning Light the next morning. Use Melatonin Onset to plan the evening.
For example, suppose you wake at 8:30 a.m. and your recent bedtime is 10 p.m. The card may show a Peak Cognitive Window from 10 a.m. to 12:30 p.m. and an Exercise window from 7:30 p.m. to 9:30 p.m. It may set your Caffeine Cutoff at noon and show Melatonin Onset from 8 p.m. You could move focused work before lunch and check your energy before an evening workout. You could skip the afternoon coffee because it falls after the displayed cutoff.
How does Body Insights estimate circadian phase?
Apple Health stores your sleep and heart-rate records. Body Insights reads the latest sleep period with enough data for an estimate. It uses the recorded sleep start, wake time, and any heart-rate samples from that period.
The lowest point in your sleeping heart rate helps anchor the phase estimate. Your sleep midpoint classifies your current chronotype pattern. Body Insights then maps the phase estimate to the alertness curve.
How are the guidance windows set?
Wake time sets the Peak Cognitive Window and Exercise window. Your recent or typical bedtime helps place bedtime and estimated Melatonin Onset. Your light prescription is the app's recommended time for bright light. It sets the Morning Light window. Caffeine sensitivity adjusts the Caffeine Cutoff when DNA data is available. Body Insights uses its standard cutoff when it is not.
New sleep data can move the phase estimate after Apple Health syncs. Wake time and bedtime can move the guidance windows. A new light plan or caffeine-sensitivity result can also move them.
How does a meal update the curve?
Tap I just ate to record the current meal time. Body Insights compares your heart rate before and after the meal. It does the same with heart rate variability (HRV). It also uses your steps during the hour after the meal. The card then refreshes the alertness curve with that meal response.
If you do not log a meal, Body Insights can use an estimated meal time later in the day. The card labels the meal time as logged or estimated while it updates.
How does the card handle missing data?
Body Insights can estimate the phase without overnight heart-rate samples. It uses wake time as the anchor. If it has no usable sleep record or bedtime anchor, the card asks for sleep data before it builds the estimate.
Related calculations
Read Chronotype from Apple Watch for the broader feature.
- See how Morning Light timing is built
- See how your Sleep Score and quality message work
- See how Morning Report plans the day ahead
See your circadian schedule
Open the Circadian Rhythm card to see when focus, exercise, light, caffeine, and bedtime fit your day.
See my timing windows