Sexual arousal causes your pupils to dilate, letting in more light -- so you see better in dim light. Like you, Unicorn, I am very visual during sex. Seeing my partner's pleasure is what pushes me over the edge. When I'm a few minutes from orgasm my eyes dilate so wide that people have asked whether I was taking drugs. (I don't. It's natural for me.) At peak arousal, I see perfectly in semidarkness and a bedside reading lamp is painfully bright. The effect disappears after I come. Over the years, I've met other men who had the same experience. I don't think this accounts for the "Dark Mondays", but it may help explain why some men always turn down the lights before sex.