You can track your sleep by simply placing your Pokémon GO Plus + on your mattress.
To prevent the Pokémon GO Plus + from falling off your mattress, please attach the included strap to it and use the clip to secure it to your pillow.
Depending on the bedding you use and where you place your Pokémon GO Plus +, your movement might not be detected properly. In such cases, placing your Pokémon GO Plus + closer to your body may improve detection.