[superuser2]

I once lived in a hacker hostel with a girl who was working 16 hour days at Tesla doing embedded systems programming. She said she honestly loved it, got lost in the flow state every day and didn't feel any effects from the sleep deprivation.

These people do exist. (Though maybe not long past early 20s).

[ams6110]

I've been in that mode -- it can be fun, as long as you like the work and don't have other responsibilities (family, etc.)

As soon as it starts causing conflict with other areas of your life, it's a fast track to burnout or nervous breakdown.

[mwfunk]

Also, everyone gets sick of everything eventually. At some point the tasks being done in those 16 hour days will cease to be fun and absorbing and interesting, at which point it becomes a living hell because you have to spend all your time pushing yourself forward, rather than being pulled forward by being fascinated by whatever it is you're working on. It's not sustainable even if there are no conflicts with other areas of your life.