How can I get him to understand that No means No?

My husband and I have been married for two years. He is 31 and Im 21 and we get along great. We rarely argue and we are happy with our marriage. The only thing that aggravates me is that lately Ive been sick from the weather I get bronchitis ever winter and Ive been taking medication from the doctor that makes me very sleepy. Anyway my husband will want to have late at night at 2 in the morning and Ill be extremely tired and not feeling well so Ill tell him we can tomorrow that Im too tired right now and he just doesnt listen or take me seriously. He just laughes and says, "Oh stop, you'll be alright." and continues to take off my clothes and does it anyway until I just have to say Forget it and go along with it. We already have almost everyday. He just laughs when I say not tonight and doesnt think Im being serious enough I guess. How can I get him to listen to me? Its really getting frustrating.