Maintenance starts in approx 7 hours from this post, and is much longer than usual as well as not taking place at the usual time.
Check Pokémon Sleep official news for the maintenance window adjusted for your local time zone.
https://www.pokemonsleep.net/en/news/313731303830353834383034363936303635/For us Britbongs that's 06:00am to 10:40am.
To ensure you can track sleep as normal, avoid updating Pokémon Sleep to version 1.11.0 prior to the maintenance period, and be sure to start sleep tracking prior to maintenance starting.
See
https://www.pokemonsleep.net/en/news/313731303833343635363037383732353133/ 