I worked pretty hard in my 20s as well, but always in a comfy way. I never pulled all nighters, I luckily worked in markets where there wasn't much need for tha

I worked pretty hard in my 20s as well, but always in a comfy way. I never pulled all nighters, I luckily worked in markets where there wasn't much need for that. I could make myself work late evenings, but never past when I wanted to sleep. If I was tired the next evening, I'd turn in early. I would say the thing that made it work was not having kids, and having autonomy over when to work and when to take it easy. I can honestly say I learned more in those years than ever before or since. My housemate worked in investment banking, much longer hours than me, with a fair bit of pressure to be awake when you want to be sleeping. The kind of job where you don't do much during the day, the MD comes in at 6pm, and tells you to make 100 slides by the next day, which he then doesn't use for a couple of days despite the juniors having stayed up all night. Grinding, and all the people who did it got visibly heavier.