Yes,Yawning comes when you body is not getting sufficient oxygen and your body need more,so when yawning comes you inhale more air,from atmosphere.
But,its not correct,that when you feel sleepy at time only.we yawn.
My personal experience,is,Every time,when i start to study,i yawn couple of times or for me its kind of habit or the proper word could be allergy is there to me.
yawning is because of lack of oxygen is what I have read when in school but as Rajni said there cannot be any particular reason. When you get bored then also you yawn!! s
