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Fitbit suggest there are a few reasons this can happen:
"WHY DON'T I SEE SLEEP STAGES TODAY?
There are a few scenarios where you might see your sleep pattern (which shows your time asleep, restless, and awake) instead of sleep stages:
taken from: https://help.fitbit.com/articles/en_US/Help_article/2163/
Best AnswerI've had the Charge 2 for 2 weeks now. I bought it exclusively to track sleep stages. Only 4 nights have actually shown sleeps stages, otherwise it's only showing sleep patterns. None of the suggested user-error help topics apply. I've seen in forums that other users have this problem of it randomly working- not working. When is this going to straighten out? Do any of the other models with sleep stages work more consistently? Currently this is completely worthless to me, and I'm getting annoyed.