Why do you feel sleepy while exercising?

Answer 1

Well, for one thing we don't even know WHY we yawn.

A quite plausible theory is that we yawn to get more oxygen because our tired bodies can't gain enough through normal breathing.

Another one is that it's a way to relieve tension and stress.

Yawning is almost always a sign of sleepiness and is common in the morning or late at night. When you work out remember to have plenty of rest, food and water beforehand.

Answer 2

When you yawn, it's your bodies way of telling you, "I need more oxygen". So if you yawn more when you're exercising it's because your heart rate is up and you're using more oxygen.

Answer 3

You didn't eat before working out and/or aren't drinking enough water.