Your watch can be set to Do Not Disturb mode and will continue to track your sleep. To do this, swipe down to access Quick Setting and enable either Do Not Disturb or Theater Mode for a quieter watch at night. Additionally, you can use the Extended Mode from Smart Battery Modes to turn off Wi-Fi and Bluetooth at night, which will eliminate any notifications (other than your set alarms). To get here, go to the Quick Settings menu and top on the battery icon to enable.