Your body clock is your circadian rhythm: the roughly 24-hour cycle that makes you feel sharper at some times of day and slower at others. On the Energy page, Body clock shows where you are in it now and how your whole day runs, from last night to tonight. It appears once NOYU has a week of your sleep.
Your day's stretches
- Waking up: the grogginess just after you wake, called sleep inertia. It usually fades within half an hour, longer after a short night or a very early start.
- Morning peak: alertness climbs through the morning.
- Afternoon dip: a natural fall in the afternoon. Research suggests it comes from your body clock, not only from lunch, though a big meal can deepen it.
- Evening peak: a second rise. Research calls the hours before your usual bedtime the wake maintenance zone: your body clock pushes hardest for wakefulness then, which is why an early night can be hard even when you're tired.
- Slowing down: alertness falls as your body gets ready for sleep, from when melatonin starts rising.
- Bed window: the hour when sleep comes easiest. See Your bed and wake windows.
After your bed window comes Night. The curve runs through the whole 24 hours, with your nights shaded behind it. It shows when you're likely to feel more or less alert, not an amount, and a day after short sleep sits lower. The soft band is your usual: where your curve ran over your recent days.
How NOYU times it
NOYU works out your curve with a well-tested model of alertness (Hursh et al. 2004): sleep refills it, time awake runs it down, and your body clock adds its daily rises and falls. It times your body clock from the times you've woken up and fallen asleep over your last two weeks, your most recent nights counting most, and your stretches come from the curve itself. Before you wake, or until last night syncs, it uses your usual times. The times are approximate, so they're shown as "about 18:45".
One late night won't shift it much, because it follows your recent nights, the way your body clock does. After a break, such as a week without your watch, it comes back once NOYU has 5 nights from your last two weeks. Rise and other apps chart their own version of your day, so their times won't match NOYU's exactly.
What moves it, and what lowers it
- Regular sleep times keep it steady. Going to bed and getting up at similar times helps it settle.
- Light is its strongest signal. Morning daylight moves it earlier, and bright light late in the evening moves it later.
- Travel across time zones shifts it gradually, at roughly an hour a day. NOYU keeps it on your body's time, so after a long trip it moves towards local time over the following week or so. When the clocks change, it does the same over a few days.
- Short sleep lowers the curve rather than moving it: after short nights, or a night without sleep, your peaks sit lower and you feel the dips more. A good night or two brings it back up. Your sleep debt, on the Sleep page, adds up the shortfall over two weeks. See Understanding sleep debt.
Planning your day around it
- Work that needs focus often goes well in a peak.
- Keep the afternoon dip for easier thinking work.
- Hard training goes by your body's physical high, which comes later than your peaks suggest. See When to train.
- Protect the time before your bed window. Dimmer light, screens away and calmer things make your bed window easier to use. Wind down, under Reminders on the Sleep page, can remind you.
Your body clock and your battery
They're separate. Your battery is your energy store, worked out from your heart rate. Your body clock is the timing of your alertness, worked out from your sleep. So after a hard day you can be in your evening peak with a low battery. See How your battery works.
