If the phone is powered on, the battery will drain. You should plug your phone in to charge when you go to bed and leave it plugged in all night, every night as standard practice. Hopefully, you've enabled iCloud sync and when the phone is plugged in to charge, the screen locked and the phone connected to Wifi, it will also backup every night, so there's little danger of every losing data, should something unfortunate happen. When you get up in the morning, you'll awaken to a phone fully charged and ready for days use.
In addition, go to Settings > Battery > Battery Health and make sure Optimized Battery Charging is turned on. This will help preserve the health of the battery.