Don't laugh, doing a PhD can be very stressful for a continued period of time.

Not in the beginning. But I, for one, felt the psychological pressure very much after year two or three. You're in an economically unstable situation, don't know if you'll be able to finish with anything worth showing (which is the nature of research), and no matter how hard you work, there are a number of relevant factors that are completely out of your control - e.g. whether or not you get enough papers accepted at relevant conferences which, over the years, has become akin to buying a lottery ticket.

The only way I kept my sanity was to set up a daily routine where I would work in the library throughout the day (a nice quiet place, as I did not have an office), and go for extended walks in the evening to air out my brain. This was what allowed me to sleep normally again at night.

Upper tier managers with crazy responsibilities are not the only people who can experience burnout. It can manifest itself in very different situations, and it doesn't hit all people the same.

Convincing yourself that your work is the only hobby you need and doing literally nothing else without regard for proper sleep or food for years nonstop is a pretty quick path to burnout. I used to think about nothing but my projects every waking moment, I'd very often skip sleep just to keep working on them and would frequently not eat anything all day because it was too much of a distraction to go buy something from the cafeteria.

These days while I still enjoy spending a lot of time working (kinda unavoidable in a PhD tbh), I make sure to leave some time for hobbies unrelated to work, which has improved my health significantly.