I'm with SlideStop on this one. Horses need REM sleep and the only time they get it is when they lay down to sleep. If they're not laying down it's usually because they don't feel secure in their environment.
I've read a couple of articles on it and dozing off and almost falling down is a classic symptom. It's also dangerous because it could happen while you're riding. Some of the remedies that were suggested are:
Making sure they have a roomy stall with plenty of bedding and horses that they feel comfortable with on either side.
Putting a mare in the pasture if the herd consists of all geldings. The jist of this being that a gelding will trust a mare to keep watch better than they will another gelding.
Good luck, I hope it all resolves itself and that Whiskey is soon laying down and sawing some logs.