I've heard that you sweat a hell lot just by sleeping. I remember watching this show and they put a normal mattress that was slept on by a normal guy on a normal day and it filled up around 1/3 of the bucket with sweat. It was a big bucket too.
I have mediumish hair, so if I shower before bed and go to sleep with my hair wet, it just looks ridiculous in the morning. Sections will be flying out in random directions, not generally what I'm aiming for.