This trigger will fire when the phone determines that you have fallen asleep of woken up (as specified).
This trigger relies on Android's activity recognition feature to determine when you fall asleep and when you wake up. This may not be accurate on some devices.
To help find a setting that works well for your own device and scenarios you can tweak the awake and sleep thresholds. If you find that the trigger is firing when you don't expect it to, you should try adjusting the thresholds until you find values that work well for you.
It's recommend to use this trigger in conjunction with a time of day constraint so it can only fire around the time that you typically go to bed/wake up. Without this you may get false triggers such as when you put your phone down and leave the room for some time you may be classified as being asleep.