For me, it's dessert, especially ice cream. I got back from a really bad time in the lab with all my samples going haywire and not making any sense when I analyzed them, as well as bad mistakes made with equipment. I felt like I made no progress and had a terrible headache by the end of the day. When it came time for dinner I got some cake, and it made me one happy camper. 
So what is it for you? Video games, beer, food, sleep, walking, chat with peers/girlfriend/boyfriend/family?
So what is it for you? Video games, beer, food, sleep, walking, chat with peers/girlfriend/boyfriend/family?