I do not do much in bed, mishap, so that's okay. It's not a chip welded under your skin.
You tell it what time you go to bed and what time you wake up. Then it assumes you are asleep during that time.
What I found interesting today was that my son's partner has one with GPS because they go running. When we were in M&S cafe, her's changed a few seconds behind mine, even though they are both the same system. My granddaughter could not decide which was right, particularly as I was wearing another watch which gave a different time.