Nobody works more than fifty hours in software. Tops. Few can work for fifty hours a week, really. You'll start lagging, or doing what a charitably call 'fake work', where you are around the office but not really being productive. (There are a thousand ways to do this.)

When you find yourself doing this, go home. You think you're doing something bad, but you are not. Your output will get better, in fact. You are not at some fortune 500 company where you need to play politics by appearing busy. You are at a startup where you need to DO GOOD WORK OR YOU WILL BE OUT OF WORK. Go. Home. Let your product do the talking.