Stand watch. Keep a look-out. Stay alert. Sentry duty.
The orders imply a solitary duty. It may require effort to stay awake and alert for the entire assigned time.
The longer the observed area remains quiet, the less attention one tends to pay to the task at hand. However, it danger does arrive, you may be in for a short time of intense activity. And if you fall asleep on the job expect a different sort of intense attention when discovered.
Sentry duty comes natural to this Black-tailed prairie dog. With his head on swivel, he checks all the quadrants for danger. Look sharp! Cartoon coyote has been spotted. (Didn’t I tell you these fellows fill in when the roadrunners take vacation?)