D&Dtools1

Even as you prepare for sleep, you feel a strange alertness in the back of your mind.
You set a specific condition under which you automatically wake up.
This condition might be anything from "If any Tiny or larger creature comes within 10 feet of me" to "When the moon is at its zenith".
The condition must be something that you would normally be able to observe if you were awake.
Thus, you can't set the spell to wake you when something happens elsewhere, or when an invisible foe sneaks into your campsite.
You awaken fully alert and ready for action.