There is no evidence to show that sunglasses can cause sleepy, you know there many reasons make you sleepy, probably you just tired. You said that when you wear sunglasses, you feel sleepy. So may be you are tired and the dark light makes you less energetic. Maybe you think about whether these days you are lack of rest or sleep. You can have your eyes a rest after one hour's work. Just look out, your eyes will appreciate you very much!
But if this just happen when you wear the new glasses, I think you may go to the shop and check whether the prescription of the glasses is too strong.
Something can cause eye fatigue, such as weather, eye strain etc. so don't play computer games or watch TV for too long.