only reason this is even an issue after only 3, 4 years of marriage (unless this question is purely out of curiosity), someone is not being 100% honest when they say "NOT TONIGHT HUNNY, I AM TIRED"...
There are several AHADEES discussing this very topic.
RC
musalman hony ka matlab yeh naheen ky hum insan na raheen!!
If my wife tells me that I will say "sure go to bed early so we can wake up early tomorrow morning" (if you know what i mean)