In stand ent...
Usually enough to keep reminding myself that deer are just beyond view and will be right along momentarily.
Sometimes just taking in God's Green Earth is enough. Plenty to take in our there. Try to pick up on subtleties: Identifying tweety-birds sounds, direction, wind variances, noises, etc.
Once whilst in the Catskills, I spent two hours trying to figure out how all of these tan moths were alive in 25 degree weather at 5:30AM flitting about everywhere! I have pics but even that took an hour to get one visible on the cell.
Sometimes, when my stand in facing the sun it's a little tough to NOT catch some Z's. It happens and yep, I'm sure there have been shootable deer that escaped me because of it.
The best thing I figure is to stay alert.