My FitBit Flex and Sleep Tracking

One question I’ve been asked on several occasions is just how accurate the sleep tracer in the FitBit devices are. I used a FitBit Ultra for over a year and have been using the FitBit Flex for a couple of weeks now. I’ve used the sleep tracker in both, but unlike the Ultra, which required an awkward armband, the FitBit Flex is always on so it is easy to use the sleep tracker.

My experience has been as follows:

  • On nights when I feel like I’ve slept pretty well, the FitBit Flex shows that by showing that I didn’t wake up very many times, or wasn’t very restless.
  • On nights when I feel like I haven’t slept well, the FitBit shows more restlessness.

It is hard to put this to the test because I tend to sleep in waves of good or bad. That is, when I am sleeping well, I’ll sleep well for weeks on end. Then, when I’m not sleeping well, that will go on for weeks as well.

Last night, however, seemed like a very poor night’s sleep from my perspective. I was tired, but restless. I felt like I was looking at the clock every few minutes and really didn’t settle down into a deep sleep. I figured that last night would be a good test to compare my perception of my sleep to what the FitBit Flex recorded. Here is what the FitBit Flex had to say:

Sleep Chart

The red lines show when I was awake. The light blue lines show when I was “restless.” Looking at this, I think it reflects very well my restlessness throughout the night.The first red line, right around 11am, was when the Little Man came into our room to tell me he needed to use the bathroom. (This is a little difficult operation for him to perform at night by himself with his cast on.) The second red line, right around midnight, was when I awoke after an unusual dream1 and jotted a note about the dream so I wouldn’t forget it.

But look at all of those light blue lines, scattered pretty evenly throughout the night. It seemed to me that I was tossing and turning for most of the night, and that is exactly what the FitBit captured. Still, a more quantitative number would be helpful. When you drill into the details for the night’s sleep you see something like this:

Sleep Details

Here, you can see that I fell asleep in 4 minutes, was awakened 17 times (some of those times count as “restlessness” as opposed to being awake, the difference being how much movement there is each time. I was in bed for 7 hours and 54 minutes and asleep for 7 hour and 16 minutes. This makes my “sleep efficiency” for last night 93%/

I have sleep efficiency data for other nights, and it seems to me that when my sleep efficiency is above 96%, I feel like I’ve slept well. When it is between 94-96%, I slept “okay.” But when it falls below 93%, I feel like I’ve had a rough night.

I think this backs up my feeling that the FitBit does a reasonably good job of capturing the quality of your night’s sleep. The resulting data matches my perceptions very well. So when I am asked how well the FitBit tracks the quality of my sleep, I guess my answer is the same as it has always been:

It does a very good job, as far as I can tell.

  1. About which I’ll have more to say in a subsequent post.


  1. Hi Mr.Rubin,
    I was wondering if you could tell me how you copied/pasted the sleep logs on this blog? I am using fitbit flex to analize my 11 year old’s sleep pattern. She had a major brain trauma a year ago today, Nov 19, 2012( not meningitis,some virus went in her brain, ) and has recovered mostly except for her sleep. I am trying to tell the doctors that she is not sleeping well and they ask me how do you know. I bought the fibit flex and it clearly says her sleep has been 88-89% past so many months. She is restless 27-38 times a night. It’s driving me crazy watching her not being able to fall into a deep sleep and not wake up fresh in the morning. It’s been a year now and I am in a mess worrying and wondering if she will crash one day due to her body not being able to heal/recover/rejuvenate due to lack of sleep. Her immune system will be affected too then …..I cant bear to go through 17 days of ICU again!
    I hope you can help.


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