Hi
Apple Watch does not recognise when you are sleeping.
To prevent notification alerts - including Stand and Breathe reminders - on your Apple Watch from disturbing you, but to still receive audible alerts for alarms and timers, turn on Do Not Disturb:
Your Apple Watch and iPhone can also be configured such that enabling Do Not Disturb mode on one device also enables it on the other. This setting also applies to scheduled operation of the mode on your iPhone - which you may wish to set for a nightly schedule, covering the hours when you are usually sleeping:
- On your iPhone, in the Watch app, go to: My Watch (tab) > General > Do Not Disturb - turn on Mirror iPhone.
- On your iPhone, go to: Settings > Do Not Disturb - turn on Scheduled and set your preferred times.
It is not otherwise possible to limit Stand or Breathe reminders to a specific time window.
If you would like to submit a feature request to Apple for any other options, then you can do so here:
Feedback - Apple Watch - Apple