You can track up to two sleep sessions per day. The day changes at 4:00 a.m. every day. If you press the "Sleep" button before 4:00 a.m., your session will be recorded as the previous day. However, if you press the "Sleep" button after 4:00 a.m., your sleep session will be recorded as the current day.
Note: These times differ if you start your sleep session by holding down the button on your "Pokémon Go Plus +" device.
For example, if you press the "Sleep" button at 10:00 p.m. on July 10th, and end your sleep session July 11th at 03:00 a.m., the sleep data will be listed as occurring on July 10th. Again, if you press the "Sleep" button at 10:00 p.m. on July 10th, and end your sleep session July 11th at 05:00 a.m., the sleep data will be listed as occurring on July 10th.However, if you press the "Sleep" button on July 11th at 05:00 a.m. and end your sleep session July 11th at 10:00 a.m., your sleep data will be listed as occurring on July 11th.