From what I've read and heard, sleeping de-stresses us because of dreams. The brain, being active as ever even in the middle of the night uses dreams, tries to 'sort out' the events that occurred during the day and associates certain imagery with it. For example, a guy who considers himself 'trapped' in a dead-end office job may subconsciously relate that to the imagery of his office's elevator: he always goes in the same elevator everyday, and come out into the same office he considers hell. He therefore may have nightmares of an elevator he can't get out of, or an elevator that 'eats' people. So every time the guy gets stressed with work, he dreams of that elevator. It becomes repetitive. As for happy thoughts, we humans usually take them for granted and don't give them much thought, which means the brain doesn't have much 'imagery' to associate happy thoughts with. Let's go back to the guy. A co-worker treating him to coffee may make him happy for a while, but even if he does it every day, the guy will probably take it for granted and doesn't give it much thought. Therefore, he probably won't be dreaming of it as well. Also, keep in mind that good dreams do not agitate people, so you won't hear them being bothered about it and asking "Why am I having that good dream again?"