Hi, @SunsetRunner , there are a number of reasons this could occur. One possible reason that is suggested by your post is if you manually start your sleep - turn on sleep mode. You must allow your Fitbit to detect your sleep automatically.
Other reasons could be if your Fitbit is unable to consistently detect your heart rate. This could be because of your sleeping position (for example if you sleep with your wrist significantly flexed) or how you are wearing your device. For sleep the device should be snugly, but not too tightly, on your wrist. If it is so loose that it flops around it will not consistently detect your heart rate.
If you spent a significant time awake, or the heart rate detection is interrupted for a considerable period your sleep times may be split into separate sections and then you will not get the detailed “sleep stages” with rem, deep, light etc.
Finally, if your Fitbit’s battery is critically low (below 20%) you may not get detailed sleep.
Sense, Charge 5, Inspire 2; iOS and Android
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