It’s just an ideal, people’s energy is a lot like battery.
It’s discharging when you’re working or studying, and It's charging when you’re sleeping or playing.
Do not let your body battery lower than 20%, that is the app’s mission.

Running on iWatch would be better.
Sadly truth is that it can only show iPhone’s BatteryLevel right now… But it was written by swift which is the newest programming language on iOS.