Body Clock

Body Clock#

What it measures#

Body Clock is a 0 to 100 index of how regular your sleep is, over 14 calendar days. It is the same idea as the regularity indexes of the sleep research: irregular sleep timing is associated with a delayed body clock and with metabolic risk (Lunsford-Avery 2018, Phillips 2017). It scores steadiness. It does not score whether your hours are the best ones for your body.

The card reads "Aligned" ("V souladu") from 80, "Slightly off" ("Mírný posun") from 60 to 79, and "Irregular" ("Nepravidelný") below 60. The number in the ring is the same whole number the words are judged on.

Body Clock: the words on the card

06080100

• Irregularunder 60
• Slightly off60 to 79
• Aligned80 and up

The parts#

Exact calculationHow each part of Body Clock is scored

Part	What is compared	Weight	Reaches zero at
Sleep length	the spread of the nights' length	35%	3h 20m spread
Bedtime	the spread of the clock time you fell asleep	30%	2 h spread
Deep sleep	the spread of deep-sleep minutes	20%	90 min spread
Wake time	the spread of the clock time you woke	15%	2 h spread
Sleep gate (Polar only)	how far your onset fell from Polar's window	20%	scored in steps

The weights are relative: the score is the weighted average of the parts that exist for you. With the Polar gate present it counts 20 of 120, about a sixth, and the other parts shrink to make room.

• Length: 100 − (the standard deviation of your nightly sleep seconds ÷ 120). A spread of 30 minutes scores 85, 1 hour scores 70.
• Deep sleep: 100 − (the standard deviation of deep-sleep seconds ÷ 54). Deep sleep swings more from night to night than total sleep, so its divisor is smaller. A spread of 20 minutes scores 78. Only nights with at least 5 minutes of deep sleep count, because a shorter episode is most likely a sensor error. This part needs 5 nights.
• Bedtime and wake time: 100 − (the spread in minutes ÷ 1.2). A spread of 30 minutes scores 75, 60 minutes scores 50, 120 minutes scores 0.

Each part needs 5 nights with the value in the 14 days. Sleep length needs 5 nights or the whole index is withheld. Nights longer than 12 hours count as 12.

How the clock spread is measured#

Bedtime and wake time are times on a 24-hour circle, so 23:50 and 00:10 are 20 minutes apart. The spread is worked out like this:

1. We find your usual clock time: the middle of your nights, nudged to the average of the nights within three hours of it.
2. Each night's distance from the usual is capped at 3 hours.
3. The spread is the root of the average of the squared distances.

The cap means that one odd night, for example one daytime sleep after a night shift among thirteen steady nights, costs part of the score and not all of it. A steady rhythm with no odd night is scored as if there were no cap.

Polar's sleep gate#

Polar watches model a window in which your body falls asleep most easily. When your Polar data carry it, we compare your measured onset with that window on each night:

Onset against the window	Night's gate score
inside the window	100
up to 30 min outside	80
31 to 60 min outside	60
61 to 120 min outside	40
more than 2 h outside	20

The gate part is the average of those nights and needs 5 nights with both an onset and a window. The window itself is Polar's model, not a measurement and not our arithmetic. It is shown on the Body Clock card as "Sleep gate (Polar)" with the note "model", and it is never scored alone. Oura publishes an "ideal bedtime" window, which we show as "Ideal bedtime (Oura)" with the same note, but it is not scored.

Minimum data and damping#

Body Clock needs 7 days of history (and at least one of sleep timing, sleep length or sleep stages), plus the 5 nights of sleep length above. With few parts the index could swing on very little, so it is pulled toward 50:

• 4 or more parts: the score as computed;
• 3 parts: it keeps 80% of its distance from 50;
• 2 parts: it keeps 60%;
• 1 part: nothing is damped, since there is nothing to average.

Example

Fourteen nights. Sleep length varies by 14.5 minutes (standard deviation), deep sleep by 6.3 minutes, bedtimes by 38 minutes and wake times by 36 minutes.

• Length: 100 − 872 ÷ 120 = 92.7
• Deep sleep: 100 − 377 ÷ 54 = 93.0
• Bedtime: 100 − 38.4 ÷ 1.2 = 68.0
• Wake time: 100 − 36.5 ÷ 1.2 = 69.6

Weighted: 0.35 × 92.7 + 0.20 × 93.0 + 0.30 × 68.0 + 0.15 × 69.6 = 81.9, shown as 82, "Aligned".

Without any deep-sleep data the three remaining parts give (32.4 + 20.4 + 10.4) ÷ 0.80 = 79.1. With 3 parts it is damped to 50 + 29.1 × 0.8 = 73 ("Slightly off"). With a Polar gate score of 90 added to the first four parts, it is (81.9 + 0.20 × 90) ÷ 1.2 = 83.

Below 60, the daily tips can say "Irregular sleep timing, try going to bed and getting up at the same times." and suggest a steadier bedtime.

The Body Clock card#

Title "Body Clock · 14 days" ("Vnitřní hodiny · 14 dní"). It hides itself when there is no score, no rhythm window and no device rows. Its rows:

• Your rhythm’s window ("Okno Vašeho rytmu"): your wake time minus one normal night (need ÷ efficiency), to your wake time. It is drawn only when the wake time passes the rule for showing a bedtime.
• Actual sleep ("Skutečný spánek"): when the shown night started and ended.
• Bedtime deviation ("Odchylka ulehnutí"): the night's start against the rhythm's bedtime, in minutes, plus later and minus earlier. Up to 30 minutes reads as good, up to 60 as fair, above that as low. It is measured against the rhythm bedtime and not the advised one, so you are not marked off-rhythm for keeping your usual hour while you owe sleep.
• Sleep gate (Polar), Hit the gate ("Trefa do okna"), Ideal bedtime (Oura), In bed in the window ("Ulehnutí v okně"), Day's alertness (Polar) ("Bdělost dne (Polar)", 0 to 10): device rows that appear only if your device sends them.

When the shown day has no bedtime or wake time, the card says "No bedtime or wake time recorded for this day. The score comes from 14-day regularity."

The typical week#

At the foot of the card is "Typical week" ("Typický týden"): one bar for each evening from Monday to Sunday. A night is filed under the evening it began, so the night from Friday to Saturday is the Friday bar. Each bar is the median of up to a year of your nights for that evening: drawn around the median middle of the night and as long as the median time in bed, with the median hours asleep at the right. Bars stacked under each other mean a steady week. A Friday and a Saturday shifted to the right is a typical weekend.

• An evening needs at least 4 nights in the year to be drawn, and at least 4 evenings must be drawn for the picture to show.
• The dashed band is "rhythm window", the window from the card above.
• The footer says "Median of 62 nights (…), hours asleep on the right."
• Hover (mouse) or tap a bar for "Friday night · 23:10 to 06:40 · 7 h 20 min asleep · 9 nights".
• The score above still uses only 14 days. A long window would mix last spring's rhythm into today's.

Week and weekend#

The card "Week and weekend" ("Týden a víkend") puts your typical weekday night beside your typical weekend night. Weekend nights here are the nights that end on a Saturday or Sunday morning. It reads up to a year of nights.

• It needs at least 8 weekday nights and 4 weekend nights with both a start and an end, and says "Weekday nights 5 of 8 · weekend 2 of 4" while collecting.
• Social jetlag is how far the middle of your night moves at the weekend (Wittmann 2006). Under 5 minutes the card says it falls at the same time as on weekdays and "your rhythm holds all week". Under 30 minutes it says it moves only a few minutes and that "your rhythm holds all week". From 30 minutes it says "The middle of your night falls 50 min later at the weekend than on weekdays. This is called social jetlag." If you also sleep at least 30 minutes longer at the weekend, it adds that you sleep less during the week and make up for it.
• Length: within 10 minutes the card says you sleep the same on both kinds of day.
• Late evenings: when your own weekday nights show it, a second sentence says what a later evening costs, for example "When you go to bed later than usual on an evening before a weekday (typically 1 h), the morning does not make it up: you sleep 45 min less." It is printed only for a clear result: at least 14 usable nights, at least 5 nights of the later kind, bedtimes 30 minutes or more later than your usual, a permutation test below 0.025 and a difference of at least 15 minutes, with the wake time following by less than half of the bedtime shift. A night more than 3 hours from the usual is a different schedule and is left out. Otherwise the card says nothing about why rather than guessing.

Frequently asked questions#

Why is my Body Clock low when I sleep enough?#

Body Clock measures how steady your sleep is, not how much you sleep. A 14-day spread of 1 hour in bedtime and wake time scores 50 on those parts even if every night is 8 hours. Weekends, shifts and travel move the clock. A single odd night is limited to 3 hours of distance, so one night costs part of the score, but a pattern costs more.

Why is there no Body Clock number for me?#

Body Clock needs 7 days of history and at least 5 nights with a sleep length in the last 14 days. Without those 5 nights the whole index is withheld. The details are under Minimum data and damping.

What does "social jetlag" on the Week and weekend card mean?#

Social jetlag is how far the middle of your night moves at the weekend. From 30 minutes the card says the middle of your night falls that much later at the weekend than on weekdays; under 30 minutes it says your rhythm holds all week. The card needs at least 8 weekday and 4 weekend nights, see Week and weekend.