I had the same issue and noticed this note;
it stopped showing sleep at 2 hrs ask it wouldn’t give any info. But at the time it quit my sleep was the time that I was waking up every 15 minutes moving. If you move too much or don’t get good sleep it doesn’t count it. You can enter your time asleep and time awake manually and it willing give you your asleep time and restless time. You just won’t get them REM or full world up of your sleep.
WHAT SLEEP INFORMATION DOES MY FITBIT DEVICE TRACK?
If your device doesn’t track heart rate, you see your sleep pattern in the Fitbit app. Your sleep pattern includes your time spent awake, restless, and asleep. Restless sleep indicates that you moved in your sleep, like tossing and turning. If your device detects excessive movement—enough that restful sleep would not be possible—your device records time spent awake.
HOW DO I SEE MY SLEEP INFORMATION?
Sync your Fitbit device each morning to see last night's sleep stats, including sleep score, on your wrist or in the Fitbit app.
HOW DOES MY FITBIT DEVICE AUTOMATICALLY DETECT SLEEP?
All wrist-based Fitbit devices automatically detect your sleep when you wear your device to bed. We recommend wearing your device in a snug wristband while sleeping; don’t wear your device in a clip or pendant accessory.
When your body is completely at rest and you haven’t moved for about an hour, your Fitbit device records that you’re asleep.
