Sleep stages

Sleep stages#

What the stages are and where they come from#

A night is split into four kinds of time. Light sleep is the largest share of a typical night, deep sleep is the slow, restorative sleep, REM sleep is where most dreaming happens, and awake is time spent awake inside the night. Your device estimates them from movement and heart signals, so treat a few minutes of difference between nights with caution.

Sources differ in how much they say:

• Some send the course of the night: each stretch with its start and end time. We draw it as a staircase on the card "Sleep architecture" ("Architektura spánku"), with the label "measured course". You can move over it (or touch it) to read each stretch, for example the clock times, "Deep" and "30 min", and open it fullscreen ("View fullscreen"). Two facts appear only for a measured night: "Awakenings" and "To first deep sleep".
• Others send only the totals per stage. The card then draws a proportional band and says: "For this night we have stage totals only, not their order, so we draw no progression."
• Some devices send no stages at all and only the length. The card then says "No sleep-stage breakdown for the selected period."

On a period view (7, 30 or 90 days) the card shows the average split over the staged nights of the window, not one night.

The stage shares printed on the tab are shares of sleep (deep, REM and light), rounded so they add to exactly 100, and the awake time stands beside them in minutes. The card "Sleep architecture · 14 nights" under the hypnogram shows one bar per night for the last 14 nights, the height being time in bed, and the shares of the night you tap.

Targets by age and sex#

Deep sleep gets shorter with age, and women keep more of it for longer. We score deep sleep against a target for your age and sex, based on published sleep studies (a meta-analysis of 65 studies by Ohayon and colleagues, 2004, and studies of women's deep sleep such as Van Cauter and colleagues, 2000, and the SIESTA study). The values by decade are our own interpolation of those studies, not figures quoted from them. Between the ages shown we draw a straight line; ages under 20 use the 20-year value and ages over 70 the 70-year value.

A 45-year-old woman is on the line between 18% and 15%: 16.5%. A 55-year-old man: between 12% and 9%: 10.5%.

Exact calculationHow your age and sex pick the target

• Your age is worked out from the date of birth in your profile on the day the night belongs to, so it moves up on your birthday. Without a date of birth the target is 15%.
• The table for women is used when your profile says female. Any other entry, or none, uses the table for men.
• Full marks need the target plus 5 points. A 40-year-old man with 15% deep sleep scores 75 on that view, with 20% he scores 95, with 10% he scores 50.

This is a score against a target, not a medical standard. A low deep share on one night is common and says little. A share that stays low for weeks is worth looking at with your usual habits (late alcohol, a warm room, irregular hours), and with a doctor if it comes with daytime trouble.

When a stage is missing#

Exact calculationHow a missing stage is handled

A stage a device never reports is not a measured zero. Some devices and older exports give no REM figure at all. A night with no REM, with the deep and light figures present, is scored from deep and awake alone: their weights (40% and 25%) are re-scaled to 100%. If deep is the missing one, REM and awake carry the part. A device with no stage columns at all, but with time awake in bed, has no stage part for that night, and the score then rests on the other parts and costs a little confidence rather than reading as "0% deep, 0% REM".

The stage part is dropped for a night when the recorded sleep plus awake is under one hour.

Stages that cannot be right#

Some devices over-report deep sleep. We do not give such a night full marks. The stages of a night are set aside when any one of these is true: deep sleep is over 35% of sleep (deep, REM and light together); deep sleep is present with no light sleep at all; or the stages add up to more than 105% of the night's length. The stage part is then missing from Sleep Quality, the "deep sleep rebound" credit in Sleep Debt is not given, and the Sleep tab prints: "This night's stage breakdown does not look plausible (for example too much deep sleep, or stages longer than the night). Your device likely mis-recorded it, so treat it with caution."

Frequently asked questions#

Is my deep sleep too low?#

Compare it with the target for your age and sex in the table, not with another person's. A share below target on one night is common. Devices estimate stages from movement and heart signals and the estimates differ from one device to another, so look at weeks, not nights. We do not diagnose anything: if a low share comes with heavy daytime sleepiness, see a doctor.

Why is there no REM or no deep sleep on my night?#

Your device did not report that stage for the night, or it reported stages we set aside as impossible (deep over 35% of sleep). A stage that is not reported is left out of the score, not counted as zero, and the other stages carry the stage part.

What deep-sleep target applies to me?#

It depends on your age and sex: men have 20% at 20, 15% at 40 and 7% at 70, women 22%, 18% and 10%, with a straight line between the ages in the table. Your age comes from the date of birth in your profile, and without it the target is 15%. The table for women is used when your profile says female, otherwise the table for men. See Targets by age and sex.

Why does the Sleep architecture card show a band and no staircase?#

Sources differ in how much they say. When your device sends only the totals per stage, the card draws a proportional band and says "For this night we have stage totals only, not their order, so we draw no progression." When a device sends no stages at all, it says "No sleep-stage breakdown for the selected period." See What the stages are and where they come from.