There is no correct reason for this or how to avoid them. Another fact is they are unavoidable and no one can really control the reflexes of the body. So yawning is unavoidable similar to other things.
Yarning is nothing but a signal of sleep. Before sleep we feel sleepy like you open our mouth and some reflections of air come out [/quote]
Oh Ok! So you mean Yawning here(with a W), that's the correct spelling and not yarning - yarn means a thread that is woven into a cloth so I was confused about the whole discussion here! Please be careful with the spellings or it becomes difficult to understand what is being said! :)
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