Pokémon Sleep maintenance will be conducted from February 23 to 24 to update to the app’s new version 3.3.0, you may not be able to start tracking sleep during the maintenance update

Pokémon Sleep will undergo maintenance from Feb. 23 to 24 to update the app to version 3.3.0. During the scheduled downtime, players may be unable to start tracking their sleep, so it is recommended to plan sessions accordingly. The update is expected to include performance improvements and adjustments tied to ongoing events and features. As with previous maintenance periods, access will be restored once the update is complete, allowing users to resume tracking and syncing their sleep data normally.

Source: Official Pokémon Sleep news and PokemonSleep.net