I'm starting to get a little bit of experience in jams so now I just try to scope things right from the begining, sleep normally (7h for me), eat quick but normal meal (like pasta or sandwiches) and being not afraid to cut features if I know I'm not gonna make it in time. Maybe some coffee to boost me up sometimes but that's all. I'm use to spend time on screen because of my study (Computer Science then Game Design), so I'm not really dead when I finish my jams, like of course you're going to work more than you do normally and it f up your week-end, but it's a good compromise to have something you are proud of at the end!
But when I started, I worked to much, was really stressed out, and the first game was quite bad, and I think quality improves when you are pretty chill.