I've had my F5 for a week now and I've got the same problem as the OP. My sleep hours show as 8 and the time is the default times I set for the do not disturb time. No actual stats are shown for the time period.
However, if I go and edit the sleep start/end time then all the stats will appear.
Tap on the sleep time counter in the My Day section of Connect. Then tap the Edit option in the top RH corner of the screen. Now select the Bed Time or Wake Time for the options to edit them.
It's something I have to do every day. I've never had accurate sleep and wake times on any of the 3 Garmin devices I've used.