Bedtime, regularity and daylight

1

What do bedtime, regularity and daylight show?

Reads built from when you sleep and the daylight your watch recorded. None of them measures your body clock.

Bedtime is a band of clock times on the Tonight page, which opens from the bedtime pill at the foot of Sleep. Home and the bedtime reminder say the same time.

Regularity is on Longevity, in the Sleep part of Body age. It shows as a number with a word: Steady, Uneven or Irregular.

Time in daylight is a vital. You find it on the Vitals page among the Activity readings, and its row reads like any other vital: a word, then your usual range.

What the clock is, and what it runs, is told in Your body clock. Here the question is only what Aera reads from your sleep times and your daylight minutes.

2

What goes into tonight's bedtime?

Your usual sleep and wake times from recent nights, and tonight's sleep need.

Your usual sleep time and usual wake time are the middle of the times you fell asleep and woke over the last 90 days. The middle is counted around the clock, so times either side of midnight sit close together.

A night counts if you slept 3 hours or more and did not mark it as travel or illness. Aera needs 14 such nights before it names a bedtime.

Tonight's need starts from your need, the one the Sleep page shows. The Tonight page lists what is added to it as Training and Paying back.

Training comes from today's Strain score. Nothing is added at 25 or below, and the most, 30 minutes, at 85 and above. With no Strain score yet, today's workouts stand in.

Paying back is for a sleep debt, the sleep you are behind on. It starts once you are 60 minutes behind and adds a twelfth of the gap, up to 15 minutes a night. These limits are Aera's choice, not taken from a study.

3

How is the bedtime band worked out?

Aera counts your need back from your usual wake time. The middle of the band is never later than your usual sleep time.

Aera starts at your usual wake time and counts tonight's need back from it. That gives the time you would need to fall asleep by. The middle of the band is your usual sleep time, or earlier when the need asks for more. It can sit at most 30 minutes earlier than that.

The band is 50 minutes wide while Aera is still learning you, and 30 once it has 44 nights. If your times usually stray more than 45 minutes from your usual, it widens to 60. The widths are Aera's choice, not taken from a study.

The band is a plan with some give in it, not a measurement of your clock.

A workout can change this. If it ended within an hour of the band opening, or later, it still adds to tonight's need but does not pull the band earlier. The reason is a 2019 review of 23 studies of exercise in healthy adults. It found no support for the idea that evening exercise hurts sleep. It said falling asleep, total sleep and sleep efficiency might suffer after vigorous exercise ending within an hour of bed.¹

That review pointed to vigorous exercise. Aera holds the band for any workout of 10 minutes or 60 kcal or more. That wider rule is Aera's choice, not the review's.¹

The line under the time says why it moved: after your workout, after a demanding day, or to win back some sleep, but only when that moved the time by 5 minutes or more. When your need alone moved it by 10 minutes or more, it says it is for the sleep you need. Otherwise it reads Close to your usual sleep time.

Switch on Bedtime reminder and Aera sends a nudge 30, 45 or 60 minutes before the band opens. It starts at 45.

4

How is regularity worked out?

Aera compares when you are asleep and awake at the same clock times, day after day, over your recent nights.

The measure is the Sleep Regularity Index. A 2017 study of 61 undergraduates introduced it. It asks how likely you are to be in the same state, asleep or awake, at two moments 24 hours apart.^2,3

Aera cuts each day into 15-minute slots and notes for each whether you were asleep or awake. It compares each night with the next across your last 14 nights. The score is the share of matching slots. It is stretched so that 100 means the same every day and 0 is what a coin toss would give.

Because it compares clock times, a late but steady schedule still scores well. Naps and waking in the night count too, since every slot is compared, not only bedtime.

On Longevity the track starts at 60 and marks good from 75. Steady is 75 and over, Uneven starts at 66, and Irregular is below that.

Those marks are Aera's choice. They sit near two numbers from a UK study of 88,975 adults who wore a movement sensor for a week: a median score of 60, and 75 at the 95th percentile.³

But that score came from a sensor, and Aera's comes from your sleep records, so 75 here is a guide, not that study's 75. A second study of UK Biobank adults, 60,977 of them, reported a median of 81, so published scores for the same kind of data differ.^3,4

Why track it at all? In that second study, more regular sleep went with a lower risk of dying, a story told in Regular sleep. It watched people rather than assigning schedules, so other habits may share the credit.⁴

5

How is time in daylight read?

Aera adds up each day's minutes and reads them against your usual. Higher is the good direction.

Aera reads the minutes of daylight that Apple Health holds and adds them up for each day. The vital's page describes it as the time your watch saw daylight.

Your usual is the middle of your last 28 days, with a band for how much the days normally swing, never narrower than 15 minutes. Days with no daylight minutes are skipped, not counted as zero.

Today is not judged while it is still going. The row reads So far today, and Aera reads the day against your usual once it ends.

Why does daylight sit beside bedtime? In a 2017 study of 61 students, the least regular sleepers also had a weaker daily light rhythm, told in Regular sleep. It was a small study of students, not a trial.²

How light acts on the clock is told in Your body clock, and studies of bright days in Bright days. Aera adds no judgment of its own about how bright those minutes were. It shows what your days contained and predicts nothing.

6

When does it show nothing?

Until Aera has enough of your own nights or days. Each read waits for its own number.

Tonight shows no bedtime until 14 nights count, and says After a few nights with your watch. If your sleep times usually stray more than 90 minutes from your usual, it says Your sleep times vary too much to name one.

Regularity waits for 14 nights with a start and an end time. Until then the Sleep part of Body age shows no number, and Body age leaves regularity out.

Time in daylight waits for 7 days of readings. Until then its row says Learning your usual.

7

What can't it tell you?

Where your body clock sits, or how bright your days were. Aera sees times and daylight minutes.

One well-known model, the two-process model, says sleep timing comes from two interacting processes: a homeostatic one, often described as sleep pressure, and one run by your circadian clock. A watch sees only the result, so Aera cannot tell the two apart.⁵

We found no published study that checks Apple's daylight minutes against a light meter, so how accurate that number is has not been published. Aera keeps only the minutes, not whether they were bright sun or a window seat.

A steady schedule fits a clock and a day that agree, but a score cannot say whether you sleep well. If you are worried about your sleep, talk to a doctor.