Startups that have recently launched and are dependent on a single product for income are going to be very intense around deadlines. If you're currently the sole / lead dev I doubt you have much to worry about. You are worth more to them than running a few hours late.

Let your cto cool off and theres no harm, then no foul.

If it's a good, popular, product then it is worth sticking with it, but of course your health should come first. Burn out over a job you don't need is not a good idea. Just try to balance things, set sensible deadlines etc so 13hr days are a rarity not the norm.

Edited for conciseness