Unlike Baraka, I'm going to assume that you know what you're talking about when you say that the apnea is under control and not the cause of this problem.
So that leads me to the body's natural rythems. If this class is in the morning, it could be that you need a break around that time, and your heart rate drops because of it. That can cause drowsiness. Also, intense focus can also cause drowsiness, especially if you're approaching one of those times a day that your body wants a rest.
Sleep usually matters two days later when it comes to performance. The night before is important but not as important as two nights before. Anything unusual there?
I'm not sure if this is a one-off or a regular occurrence for this class. If it's a regular thing, perhaps you can excuse yourself to the bathroom to get your heart rate back up. If it's a one-off, see what happens next class.
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