Apple Watch recording sleep data when I’m not even wearing it

My watch recorded my best night of sleep ever last night! But I wasn’t even wearing it… strangely, it even broke my sleep up into very specific stages (according to the sleep app, it’s the most deep sleep I’ve (not) gotten in one night).

This is a bit disturbing, as collecting sleep data is the primary reason I bought my watch. Is Apple just making this data up?