Hello @Kristina729 ,
this is the automatic notification that guests receive depending on your property's time zone.
If it is nighttime at your property's location, guests will receive this message automatically.
Unfortunately, there is no setting to specify a specific time when you won't be able to respond to guests (so you have 24 hours to respond).
But I think it would be helpful if hosts could specify a time when they are unavailable (if they actually live in a different time zone than the property). I can recommend you to share your feedback about this question using feedback form: https://www.airbnb.co.uk/help/feedback so that the specialists can look into this issue.
Kind regards.
Elena.