Try the usual first. Restart by holding the top left button until watch shuts down or top right button if you don't have buttons on the left. Connect watch to Garmin Express on a computer and check for any updates.
You can't install the sleep function. It is always there. If you swiped it away from My Day you can scroll down, click Edit My Day and enable any removed cards.
What does the body battery graph look like? Post a screenshot.
