About the sleep chart numbers
A few places where the chart and your memory of the night won’t quite agree — and why each one is on purpose.
The first two are the ones you’ll actually notice. The rest are here so you can check.
After what the shapes mean? That’s How to read your sleep chart →
The bars show time in bed, not time asleep
Each bar covers the stretch you had Sleep Cycle recording — lights out to alarm off. Sleep Cycle counts the time you were actually asleep separately, and that number is always smaller. Usually by around an hour a night.
So a row that reads 08:30 is closer to seven and a half hours of real sleep. If you want that figure, your export already has it, in a column called “Time asleep.”
Why it’s drawn this way
This chart is about when you sleep and how steady that is, not how much. Drawing only the asleep minutes would punch small holes through every bar wherever you stirred in the night, and the shape you came here to see would disappear.
Twice a year the clocks change and one row won’t add up
The chart is a wall clock: 24 columns, one per hour. Two nights a year the day isn’t 24 hours long.
- Clocks back. 1am happens twice and both of them land in the one 1am column, so the columns come up an hour short.
- Clocks forward. 2am never happens, but the 2am square is filled in anyway. Leaving it blank would put a gap through the middle of the night that looks like you woke up.
The total at the end of the row is your real time in bed either way. On those two rows it just won’t match the squares beside it. Nothing is wrong.
Each row runs from 6pm to 5pm the next day
Rows are cut in the early evening rather than at midnight, so an ordinary night sits whole in the middle of one row instead of being sliced in half and read as two.
The trade-off
A sleep that crosses 6pm gets split instead: the hours before it finish one row, the hours after start the next. If you sleep during the day, some of your sleeps will show up in halves. Each row’s total is right for the part it holds — only the picture is divided.
One rare overlap
A very long sleep that starts in the afternoon and runs past dawn, landing on one of those two clock-change nights, can put its extra hour in the row above. One hour, twice a year, and only if your nights look like that.
Times stay exactly as Sleep Cycle recorded them
Sleep Cycle writes down the local time wherever you happened to be, and that is what gets drawn. Nothing is shifted back to the time at home.
So a week away shows up as a week of sleep at different hours — which, in clock terms, is what it was. It also means your chart looks the same to anyone you show it to, wherever they are.
A night that appears twice is only counted once
Exports sometimes list one sleep twice, with the same start and end time. The second copy is ignored, so that night isn’t doubled.
And if two sleeps ever lay claim to the same hour, that hour stops at full. An hour can’t hold more than an hour.
It reads Sleep Cycle exports, and only the times
- Chart My Sleep works with Sleep Cycle exports. Other sleep apps write their files differently and may not be recognised.
- Only the start and end of each sleep are used. Sleep quality, heart rate, snoring, steps, mood and the weather are all left out.