For years, this horrible app has been flubbing up recorded sleep data.
Immediately after syncing, my device will show (for example) that I slept from 11pm to 7am, with no fine-grained data like REM, or awake. The colored graph will be mostly featureless.
A few hours later, when I check back, many things change. For example, it will show new (incorrect) sleep and wake times, and the data will now show wakefulness and REM sleep.
This problem is consistent and happens everyday.