That happens because when you lie in a reclining position and read, your body/vitals relax and the sensors think you are asleep. The best way to prevent this is to a) turn off sleep mode immediately, b) open the health app to check your sleep stats, c) move your watch arm occasionally while you are reading and d) sit up a little higher. You can only delete a sleep cycle if it registers as two separate sleep sessions. If it registers as a long break or a big chunk of orange, you cannot delete part of it, and there is no space for notes. People have been asking to at least have notes for quite a while.
