Hi
It is normal for Apple Watch to turn on automatically if it is placed on the charger whilst it is turned off.
Rather than turning your watch off each night, you may find it convenient to charge your watch nightly, overnight.
The watch cannot be overcharged and the battery will not suffer any harm from regular charging. Charging will stop automatically when the battery is fully charged and start again if and when required due to ongoing battery usage.
Follow Apple's safety and handling instructions and also note that:
- It is not possible to turn the watch off whilst it is on the charger and connected to power - this is normal.
- It is also normal for your watch, power adapter and charging cable to become warm when connected to power.
More information:
Check your battery and charge your Apple Watch - Apple Support
Keep Apple Watch within acceptable operating temperatures - Apple Support