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If it's not syncing then it may have tracked your sleep ok but just cannot update your dashboard with the information. Luckily the One can hold up to a week of data between syncs so hopefully your sleep will reappear when it syncs.
First thing to try is to restart your phone or computer (whatever you sync with) and then restart your One using this procedure:
https://help.fitbit.com/articles/en_US/Help_article/1186/
Give this a few tries as it doesn't always work.
If it still won't sync after that then there are some more suggestions from fitbit here:
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@Gillyr43 did you get this sorted in the end? I notice it's been a week and you'll start losing data soon.
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