Probably my favorite moment of going to bed is that moment when, after sliding into a bed made with clean sheets, the cool material has warmed exactly half-way to my body temperature from ambient. If there was some way to keep the sheets at that temp, everything would be right with the world. Maybe small ventilators, or water pipettes in the material, or...I dunno. I just know that at that moment, lying on my back with the sheets slowly warming, I begin to approach sleep. Demeter and Ansate are absolutely right; if you're having trouble sleeping, try changing your bedclothes.

Some nights it is chilly enough out that if I move around in bed, the part of the sheets that weren't touching me are that temperature. I prefer it to be chilly when I'm in bed; I'd rather sleep under a comforter than under a thin coverlet. Of course, I'm a tad heavy, and my body doesn't cool itself as well as you endomorphics (physics...surface area / volume...)