Startups are super hard. As an employee what you need to evaluate is your skin in the game.
With the kind of efforts you are putting up, what’s the upside for you? Are you paid brilliantly well, are you having esop that can help you make life changing money?
There comes a time in life where even if you are not working at startup there is no life outside work because all your friends/family/colleagues are busy building their life, so you will eventually end up getting alone or spending very time with all.
Among all you evaluate what you want? If you are in age of 26-32 yrs, this is the time when you have energy/enthusiasm to hustle but as you grow from here you will start having family, responsibility of parents/children etc and that is when you need time and money both. So if you are in a state where you can make great money today buy hustling you are actually safe guarding your future where you will have both.
Now answering the other part, do take small breaks, once a quarter/six month get away for a small vacation/trips.
Play sports/gym/road trips once a month, these are things that will keep you away from getting mentally saturated and burned out.
Rest, hustle hard!
If your startup is not rewarding you well, switch as well. Hustle only when you know there is great upside to it.