The later part of my day is often the most productive - fag break meetings have initiated most of the best work we've produced by having an informal chat before something becomes a thing.
Working hard is genuinely admirable, I did startups and put in some serious deathmarch shifts, but I honestly believe that if you're delivering what you signed up to do, and using any leftover time productively, you're making a positive net contribution and probably becoming a better developer as a result. It's a win-win and you won't burn out.
Write beautiful, elegant code, deliver everything you've promised you will, the rest of the time just enjoy doing something you love.