When you set your alarm every night and lock your phone screen with the app open, it keeps working in the background, which means that it can somewhat reduce your battery charge. We advise you to charge it to avoid possible inconveniences, such as, for example, that the alarm does not sound because phone switched off.
Leaving the phone plugged in will damage my battery!
Battery degradation has more to do with the way we charge than with the time the device is connected. Try to avoid short charges and drain the battery until it is empty.