This isn't startup specific! I work on projects with long development cycles, and rapid prototyping outside of work is very therapeutic. You can amplify the healing power with the right tools: in my case, Python and Git. There's nothing more satisfying than creating something in a few days, or extending an existing project to do more. Be sure to let someone know you did something! Thankfully my girlfriend listens to me babble about my minor wins, even when they sound ridiculous or incomprehensible.