Instinct 2 camo records sometimes too much sleep?

Hi,

Every once in a while my instinct 2 will record too many sleep, I mean like more than 24hrs.
For instance: Today was 31hrs35min but only a total sleep of 33min.

Most of the time it is spot on regarding sleeping times but this issue seems to appear more regular lately (like 3 times in the last month).

Does anyone know what can be the issue here?
The watch is now 12months old and was purchased new.