Does the Fitbit really measure sleep?
The Fitbit (and similar devices) has an accelerometer. It measures change in motion. That’s all. It does not measure brain waves, or breathing, or blood oxygen. It can tell whether there is a change in your movement. So if you are lying quietly and then start moving, it senses that. The Fitbit probably thinks you are asleep when you are not moving. And it thinks you are awake when you are moving. But, people lie awake and quietly in bed. People move when they are asleep. Thus, the Fitbit really does not know when you are asleep. Research has shown that the Fitbit does not measure sleep reliably.
The claim that the Fitbit can tell what stage of sleep is, of course, nonsense. Brain waves (and eye movement and muscle activity) determine sleep stages. An accelerometer figuring out what sleep stage you are in – absurd!
Is the Fitbit useful at all when it comes to sleep?
So, is the Fitbit useful at all when it comes to sleep? Yes, it can tell you when you went to bed and when you got out of bed. That is important information, because the average American does not get enough time in bed. This makes us tired and sleepy. It also makes us cranky, leads to more mistakes, and causes weight gain. Also, too many of us do not have regular sleep habits. On the other hand, the elderly often get too much time in bed and this leads to poor sleep (insomnia). It is important to go to bed at about the same time and get up about the same time every day. It may be all right to get an hour or so extra on weekends (and to go to bed a little later). Regular sleep habits and enough (but not too much) time in bed are good for our health.
Use the Fitbit to determine how many steps you took. Use it to keep to a regular sleep schedule. Do not rely on it to tell you when you are asleep. And certainly do not expect it to tell you your sleep stages.