# How does my smartwatch know how I sleep? **URL:** \ **Category:** Factual Questions\ **Created:** [July 1, 2019, 1:09pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341 "2019-07-01T13:09:43Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [July 1, 2019, 1:09pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/1 "2019-07-01T13:09:43Z")
If I wear my smartwatch (Huawei\* Watch 2 Sport) to bed, it will show me a report on my sleep. It shows light sleep, deep sleep, and REM sleep over time. How does something I wear on my wrist know this? * * * \*Hopefully it is not sending my sleep data to the Chinese government. ---
**Author:** ![TubaDiva](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tubadiva/32/507_2.png) [@TubaDiva](https://boards.straightdope.com/u/TubaDiva)\ **Post date:** [July 1, 2019, 1:18pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/2 "2019-07-01T13:18:09Z")
It doesn’t know how much you sleep, but it does know when you’re moving around. So if you’re lying absolutely still for a period of time it assumes you are asleep. If you’re moving, it assumes you are not. Jenny your humble TubaDiva ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [July 1, 2019, 1:22pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/3 "2019-07-01T13:22:46Z")
> [@TubaDiva](#): > > […]if you’re lying absolutely still for a period of time it assumes you are asleep. If you’re moving, it assumes you are not. But how can it tell the difference between light, deep, REM? I figured you needed an EEG for that. ---
**Author:** ![dofe](https://avatars.discourse-cdn.com/v4/letter/d/7ba0ec/32.png) [@dofe](https://boards.straightdope.com/u/dofe)\ **Post date:** [July 1, 2019, 4:35pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/4 "2019-07-01T16:35:53Z")
> [@CookingWithGas](#): > > But how can it tell the difference between light, deep, REM? I figured you needed an EEG for that. The Fitbit tracks sleep stage based on heart rate variability that occurs during each stage – I’d imagine your smartwatch uses a similar methodology. ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [July 1, 2019, 5:10pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/5 "2019-07-01T17:10:43Z")
> [@TubaDiva](#): > > It doesn’t know how much you sleep, but it does know when you’re moving around. So if you’re lying absolutely still for a period of time it assumes you are asleep. If you’re moving, it assumes you are not. > > Jenny > your humble TubaDiva My fitbit can tell the difference between sleeping and taking the device off and setting it down overnight. But it isn’t using heart rate to distinguish, because I have a cheap, bottom tier fitbit that doesn’t have that feature. My guess is that it’s looking for _some_ movement, but not too much. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [July 1, 2019, 6:24pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/6 "2019-07-01T18:24:26Z")
My Fitbit also doesn’t do heartrate. I specifically got it to track sleep cycles. It sort of knows when I’m “restless” at night. But it misses a lot. All too often it’ll think that I’m asleep most of the day and other stupid stuff. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [July 1, 2019, 6:29pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/7 "2019-07-01T18:29:59Z")
Have you tried comparing it to an actual EEG of your sleeping brainwaves? If you had I think you would have discovered that the answer is “It makes a rough guess.” ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [July 1, 2019, 7:48pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/8 "2019-07-01T19:48:32Z")
> [@naita](#): > > Have you tried comparing it to an actual EEG of your sleeping brainwaves? If you had I think you would have discovered that the answer is “It makes a rough guess.” An EEG is not something you just casually decide to go do. I had one 40 years ago, so no, I haven’t compared it. ---
**Author:** ![Lips\_Obsession](https://avatars.discourse-cdn.com/v4/letter/l/bbce88/32.png) [@Lips\_Obsession](https://boards.straightdope.com/u/Lips_Obsession)\ **Post date:** [July 1, 2019, 7:48pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/9 "2019-07-01T19:48:32Z")
I am not a calm sleeper - I toss and turn quite a lot. My fitbit rarely tracks my sleep properly. It thinks I only sleep an hour a night, or maybe a tiny bit more. I usually sleep 6-7 hours a night. ---
**Author:** ![cmkeller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmkeller/32/118_2.png) [@cmkeller](https://boards.straightdope.com/u/cmkeller)\ **Post date:** [July 1, 2019, 8:02pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/10 "2019-07-01T20:02:05Z")
It has a direct connection to Santa Claus, who sees you when you’re sleeping. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 1, 2019, 10:21pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/11 "2019-07-01T22:21:20Z")
[How Sleep Tracking Works](https://sleep.urbandroid.org/documentation/core/background/#understanding-sleep-graphs) for _Sleep as Android_, an Android sleep tracking app: > [@](#): > > In deep sleep, your muscle movements are suppressed – the sleep graph gets nearly flat. > During light sleep, you tend to toss and turn which manifests as significant peaks in the sleep graph. > > Your body movements are directly related to your current sleep phase. In general, the more movement the lighter the sleep. ---
**Author:** ![Yllaria](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yllaria/32/3452_2.png) [@Yllaria](https://boards.straightdope.com/u/Yllaria)\ **Post date:** [July 1, 2019, 10:28pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/12 "2019-07-01T22:28:25Z")
> [@cmkeller](#): > > It has a direct connection to Santa Claus, who sees you when you’re sleeping. Okay, that made me smile. ---
**Author:** ![Omar\_Little](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omar_little/32/269_2.png) [@Omar\_Little](https://boards.straightdope.com/u/Omar_Little)\ **Post date:** [July 1, 2019, 10:35pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/13 "2019-07-01T22:35:55Z")
NM ---
**Author:** ![Omar\_Little](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omar_little/32/269_2.png) [@Omar\_Little](https://boards.straightdope.com/u/Omar_Little)\ **Post date:** [July 1, 2019, 10:37pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/14 "2019-07-01T22:37:41Z")
Well it was manufactured by one of the leading espionage companies, it probably tracks more info about you than just sleep habits. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [July 1, 2019, 10:45pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/15 "2019-07-01T22:45:00Z")
> [@CookingWithGas](#): > > An EEG is not something you just casually decide to go do. I had one 40 years ago, so no, I haven’t compared it. I wasn’t expecting you to. It was just a rhetorical device to point out that the smartwatch doesn’t actually know how you sleep. It just makes a rough guess based on what it can detect. I.e mainly how much you move your arm, along what axes, and for some watches your HR. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [July 2, 2019, 4:49am UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/16 "2019-07-02T04:49:00Z")
I got my Fitbit to monitor my sleep. I have sleep apnea so I thought it would be useful. I have found it not very accurate. It has counted laying in bed reading as being in deep sleep. There have been nights when I got up because I was not sleeping, the fitbit recorded me sleeping all night long. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [July 2, 2019, 6:06pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/17 "2019-07-02T18:06:24Z")
I don’t know how this particular watch works. But during REM your body is mostly non-moving, NREM involves more tossing and turning. And typically you cycle between the stages of sleep (e.g. awake-1-2-3-2-1-REM-1-2-3…). Maybe it makes a guess based on the cycling rate? > [@CookingWithGas](#): > > An EEG is not something you just casually decide to go do. I had one 40 years ago, so no, I haven’t compared it. How do you mean? It isn’t invasive. I suppose if you’re billing for insurance you can’t just request it, but if you have access to one it’s a trivial procedure. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [July 2, 2019, 6:36pm UTC](https://boards.straightdope.com/t/how-does-my-smartwatch-know-how-i-sleep/836341/18 "2019-07-02T18:36:07Z")
> [@thelurkinghorror](#): > > How do you mean? It isn’t invasive. I suppose if you’re billing for insurance you can’t just request it, but if you have access to one it’s a trivial procedure. I mean that you need a doctor’s orders to get an EEG, and the way they do the test depends on what the doctor is looking for. I don’t know what you mean by “trivial.” When I had one, they had me stay up late at night and get up early so I would be good and tired. They needed an EEG while I was asleep. I can’t remember how long I was there, but it was more involved than getting a haircut.