You wake up feeling perfectly fine.
Then your wrist says you slept badly and gives you a score out of a hundred.
By the time you reach the kitchen, you feel tired.
That is worth guarding against.
Your watch did not watch you sleep.
It built a story out of very little information.
How does a watch know you fell asleep?
It knows two things about your night.
It knows how much your wrist moves, and what your pulse is doing.
Your arm goes still for a long stretch.
Your heart rate settles below its daytime level.
Software then decides that you are asleep.
That is enough to spot when you went to bed and when you got up.
It is not enough to know what your brain was doing.
A sleep laboratory reads brain waves with sensors on your head.
Your watch has none of that.
How good are the deep and REM numbers?
They are the softest numbers on the whole screen.
Deep sleep and REM are stages defined by brain activity.
Your wrist cannot see brain activity.
It infers the stages from stillness and heartbeat patterns instead.
Two brands watching the same night often report different amounts of deep sleep.
Lie awake very still and some devices will record that as sleep.
So the total is roughly useful and the breakdown is mostly decoration.
What is worth looking at instead of the score?
Your bedtime, because that is the part you can change.
Your wake-up time, and how much it swings between weekdays and Sundays.
Your total hours across a week, rather than any single night.
Your sleeping heart rate, watched as a trend.
When is a bad score not a problem?
When you woke up feeling rested.
That beats any number on a screen.
A low score after a wedding or a sick child is just an accurate record.
One rough night needs nothing beyond an early bedtime.
If the score has started to make you anxious, turn the feature off.
Worrying about sleep is a reliable way to sleep badly.
Things to Remember
- Trust the bedtime and wake time, not the stage breakdown.
- Judge your sleep by how you feel when you wake.
- Look at a week of hours, never one night's score.
- Fix your bedtime first, since that is the part you control.
- See a doctor about lasting daytime tiredness or loud snoring.