Wearing your apple watch to sleep can actually benefit you, if you know about the features it offers. Your apple watch can track the amount of hours that it reads you are actually sleeping. It also tracks your heart rate while simultaneously tracking your sleep. Using watchOS 9, it has new and improved features that allow it track track and read what type of sleep you are in and for how long. Such as REM, Deep, or Core. It also detects when you wake up from tossing and turning and how often. To be able to do all this though, you will have to set it up with your watch. You can go to Sleep Mode and start there, if you're unsure where to start, the health app has tutorials to help you. Once you set this up, the next morning after you slept, apple will send you a summarized notification of your night and all the it tracked. If for some reason you didn't get that notification, you can go to the health app and click browse, once you see sleep, just click it and all your data will be there. However, not only does wearing your watch to sleep have this benefit, it can also wake you up with a hepatic alarm!