It shouldn’t be doing that. Try deleting and reinstalling the Sonos app.

If you, or someone else, in your Household are using Soro, or similar 3rd-party App to schedule/activate the ‘night mode’ feature, then you/they just need to switch off the schedule, or disable the option via whatever App you/they are using (it’s probably using the Sonos API interface?) The Sonos App itself has no such option to automatically-set/disable/schedule the night-mode feature. 

Ironic: it is an oft-repeated request for this exact feature, and somehow the OP has that feature and wants to disable it. LOL.