AMATERASAMATERAS › How it works
How does Smart Alarm estimate an easy moment to wake?
The phone’s accelerometer measures movements such as turning over. You move more in light sleep and less in deep sleep, so during the few minutes before your alarm time (5 minutes by default), it rings at a moment you keep moving. If you don’t move, it rings at your alarm time. It does not tell REM and non-REM sleep apart.
Why wake you in light sleep
Being woken from deep sleep is known to make the grogginess right after waking (sleep inertia) stronger1,2. Deep sleep comes mostly in the first part of the night. Large body movements also cluster where sleep stages change3. So a moment you move is used as a sign that your sleep is light.
However, there are no high-quality studies (randomized controlled trials) yet showing that smart alarms make waking easier. The app makes no such claim; it shows how it works and where its estimates fall short.
What it measures
- Movement: the accelerometer is read 25 times a second, and every 30 seconds it summarizes the amount of movement, the number of peaks, and changes in the phone’s tilt.
- Sound level (optional): only if you allow the microphone, it counts sounds such as turning over. No audio is recorded or saved. Continuous sounds, such as traffic outside, are not counted as movement.
- Records are stored on your device.
How it estimates sleep depth
- It compares each 30 seconds of movement with your usual movement (past nights and tonight) to get a movement strength. Movement below three times the sensor noise is not counted.
- Small movements damped by a soft futon or mattress are also picked out from that night’s noise level.
- From movement strength and how often you moved in the 5 minutes around, it estimates sleep depth.
- A phone’s accelerometer alone cannot tell sleep stages (REM, non-REM) apart. So it shows no stage names, only a continuous graph of estimated depth.
When it rings
| Wake window | From 5 minutes before your alarm time until your alarm time (0–15 minutes in Settings). |
|---|---|
| It rings when | You seem to be waking (awake for 1 minute in a row); or in the last 4 minutes there were two clusters of strong movement, or one sustained movement. After a single turn-over, it waits 90 seconds and rings if you move again. |
| It does not ring | Because of movement within 3 hours of falling asleep (when deep sleep is common). If the phone is on the floor or tracking is interrupted, it waits until your alarm time. |
| Alarm time | It always rings. A backup alarm (the system alarm on iOS 26 and later, a notification before that) is scheduled every night, so it rings even if you forget to tap “Sleep”. |
| Waiting later | Off by default. If turned on, it waits up to 10 minutes past your alarm time while you sleep deeply. |
What it cannot know
- A phone alone cannot confirm which mornings were really light sleep.
- The wake window is short (5 minutes), so on many mornings you don’t move in it and it rings at your alarm time.
- It is not a medical device and cannot be used to diagnose or treat any condition.
Sources
- Tassi P, Muzet A. Sleep inertia. Sleep Medicine Reviews. 2000;4(4):341–353.
- Hilditch CJ, McHill AW. Sleep inertia: current insights. Nature and Science of Sleep. 2019;11:155–165.
- Aaronson ST, Rashed S, Biber MP, Hobson JA. Brain state and body position: a time-lapse video study of sleep. Archives of General Psychiatry. 1982;39(3):330–335.
