I used to have this problem also, nearly every workout. I can think of two things that might have had a direct impact on fixing the problem for me.
Adequate hydration is one. I have been drinking at least 1.5 gallons water each day (coming largely from shakes). Last time I yawned a lot during a workout, I was also very dehydrated. You guys might want to try this and see if it helps. Many people think they may be drinking a lot of water, but in reality are not consistent about it. The other is as already mentioned, getting plenty of sleep! A mild preworkout stimulant might make a difference for some.
It can be a really annoying problem, lol!
-Cardinal