Most Fitbit activity monitors come with built-in sleep monitoring that works automatically — there's no need to tell your Fitbit you're going to bed, as it will just sense it. Fitbit uses several methods to detect that you're asleep. When you haven't moved in about an hour, it assumes you're asleep, but it confirms this by measuring small movements (like rolling over in your sleep) as well.
If there's a heart rate monitor built into your Fitbit, it combines heart rate and movement data to develop a more accurate picture of your sleep cycle.
Fitbit doesn't just track your overall sleep, but your sleep stages as well. A complete sleep cycle contains several stages, such as light sleep, deep sleep, and REM (rapid eye movement) sleep.
#fitbit How a Fitbit is able to accurately track your sleep, and how to check your sleep stats
Most Fitbits will track your sleep automatically with a combination of movement and heart-rate monitoring. You can also check your sleep stats later.