These are all in the Settings app.
- Battery > Low Power Mode ON
If Low Power Mode didn't exist, this list would be much longer. It disables most iOS background features like email fetching, app refreshing, app updates, and iCloud photo syncing. It automatically turns off once your battery is charged to 80%, so you may have to turn it back on again. - Do Not Disturb > Do Not Disturb ON
Do Not Disturb will prevent the screen from turning on whenever a notification comes in. Double-check the settings here, or you may not get an urgent phone call.