How does my Fitbit know how much I've slept?

My doctor has me wearing one of these things to track my sleep as part of my insomnia treatment. It hasn’t been explicitly said that I won’t get this or that med if I’m sleeping too much, or so little that the med isn’t working, but I’m pretty sure some of that is going on.

The thing is, the Fitbit jibes fairly well with my subjective experience, and I’ve even learned some things-- for example, stretches of sleep, even relatively short ones that are accompanied by vivid dreaming are the most restful, which is counter-intuitive, but it works that way.

I assume it measures sleep partly by movement, and partly by pulse rate-- and I am a very still sleeper, to the point of sleeping very safely on top bunks in the middle of a room without safety bars, or even on the third tier of narrow train sleepers; but awake, I’m fidgety. So I suppose my wearing the Fitbit all the time allows it to collect information on differences between me awake and me asleep, which is how it’s not fooled by my lying down awake.

Still, there are times I’ve been lying down tired, and I startle, and am not sure whether I’ve been actually asleep. If I check my phone (ie, the Fitbit app), it’ll tell me one way or another.

Judging also by how tired I am during the day, and how I feel at bedtime the next night, the Fitbit seems to be pretty accurate.

Can it really be as simple as motion and pulse rate? I guess sleep seems like such a profound change (especially to an insomniac), that using such simple things to measure it feels too easy. What else is going on that I haven’t figured out?

Google is failing me here-- all I get are more guesses on third party websites. Fitbit seems to be guarding its secrets.

You are correct that it relies primarily on your movement. And yes you can fool it, but why would you be trying to fool your fitbit? What purpose would it serve?

Yes, it relies on movement. If I wake up, but remain very still, mine will think I am still asleep until I actually get up.

I don’t have any intention of fooling it, and probably couldn’t if I wanted to. Just wondered how possible it were.

I tend to startle awake, even if I fall right back asleep, so my sleep is usually punctuated by lots of thin red lines.

I have an Apple Watch and did some research into getting a sleep tracking app last year. I didn’t because in my case I don’t have serious sleep issues and thought I track enough with my watch now, I don’t need more.

They’re basically a cheaper proxy measure in lieu of doing a full on sleep study. As @Omar_Little says - it’s movement and heart rate. The assumption is that if you’re moving you’re awake and is you’re not you’re sleeping. One issue is they assume that all movement comes from you. If your bed partner moves, it assumes you’re awake, if a pet jumps on the bed, it assumes you’re awake.

Yes, they can be tricked, but I’m trying to imagine you laying still in your bed absolutely motionless just to trick your watch. Bizarre trickery to say the least.

Although Fitbit has a sleep tracker built in, Apple does not. From what I’ve read, that’s a very deliberate choice by Apple. They believe that at best, the motion & heart tracking apps are a loose approximation of your sleep and not accurate enough for them to officially endorse.

As far as your google skills go, try “do smart watch sleep trackers work?” And you will get more hits than you can read.

Probably similar to what the Whoop does. I just got one a week ago and it seems to pick up on sleep and exercise pretty accurately. Here’s a link to their marketing/science. There’s a link in that article to the U of Arizona study that was done recently to analyze and compare the Whoop to the full polysomnography test that is done with ekg and other monitors.