I would like to ask that i always feel sleepy whether it is class or room and when i try to avoid it my legs and arms start paining too much i feel so sleepy that i frequently sleep in all lectures?

Sleepy and sleepier: You no doubt have had a thorough medical evaluation including sleep screen and blood work. If there is associated hallucinations with sleep initiation/termination, sleep paralysis, or involuntary muscle weakness with intense emotion, consider narcolepsy. Pain and discomfort in your extremities may be due to restless leg syndrome - an easily treatable and common problem. Is sleep restful?

Apnea?: One possible explanation, amongst many could be lack of proper oxygenation due to obstructive sleep apnea; many people who suffer from this report falling asleep during the day, at work or in classroom, when you should be alert. Are you overweight? Snore? A consult with an md would be of certain benefit.