My girlfriend gets very sleepy while exercising and yawns all the time. We have searched online but couldn't find any answers except a few brief mentions of several possibilities, like heart or blood pressure problems (she has very mild anaemia), sleep depravation (not at all, she sleeps a lot - she can sleep 11 hours during the night and still take a nap in the afternoon) or fatigue (also not the case, on the contrary, she has been having a very sedentary life lately, unfortunately).
Any ideas on what could be the cause for this sleepiness that strikes her just a few minutes after she starts exercising?