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.