I hear this a lot. But I'm not sure if this is always true. I chose an education path that required a much greater amount of work than other paths I could have taken. I would spend weekends working on homework and projects when others were out having fun. But as I start my career, it seems to have paid off (so far). I have a better work/life balance than many of my friends due to sacrificing some free time earlier in life.
Now I wonder: if I work hard in my 20s, can I retire a decade earlier that I otherwise might have? Or (more likely) can I save up enough that I can start spending my time working on projects that are entirely self-driven? Saving and working hard early in life can also result in a lot more time to spend with family later on. In other words, there is a tradeoff between voluntarily working harder earlier in life and effectively being forced to work hard later in life. Sure, you could be struck by a meteorite today, and you would never reap the reward from all of the work that you put in, but the much more likely scenario is that if you don't put in enough work in the front end, you will end up working 9-5 at 70 years old. So I can see many possible situations arising later in life where I think "I wish I spent more time working".
Sure you could argue that you get more free time later if you give up free time now but then you enter into the difficulty of quantifying the quality of the free time. When you are young, active and healthy you might be able to enjoy your free time more or perhaps having more money later in life allows you to do things you enjoy more, either way I feel balance is the key.
Do you really want to spend your most productive, healthy and free/uncommitted years working too much just to have more time when your body is starting to weaken and many of your dreams start to become impossible?
Do you want to spend less time with your children for the chance to have more time for your grandchildren?
The "working a lot" was mostly the last couple years of my Ph.D. where I would frequently pull all-nighters to finish everything I needed to, but I'm glad I did that now. It's not nearly the optimal path for rapid wealth accumulation, but it's a good balance between having work that I enjoy and having enough leftover time to work on startup projects and hang out with my family.
Early in life, I had a small amount of artistic success (music). Though I probably couldn't have gone big with it, I did enjoy some amount of regional fame. Hard work, definitely an accomplishment, and I built several friendships out of it that have lasted over two decades (as well as lots that did fade away in time).
Similar thing with a startup I cofounded several years ago--the problem we were solving had me out in the public eye for a while, and I ended up building friendships with lots more people...people I'm still friends with.
And you know, if not for the relationships I build, some of which turned into lasting friendships, I wouldn't have had the success in the first place.
Nothing goes anywhere in life without people around you, helping you out. If you don't treat those people as the most important part of whatever you're doing, you're doing it wrong.
This is one of the fundamentals of leadership, and leadership starts with leading your own life. One of the other fundamentals is awareness -- self-awareness and awareness of others, which these comments seem to be touching on.
The issue here is we do not know the true spectrum of outcomes with and without working. I assume many people who say "I wish i worked less" on their deathbeds presume that all the trappings of their work would come with.
I would trust it more if people paired what they'd give up with what they want to gain. ie if they said "Putting my daughter through college wasnt worth it, I wish i worked less". Or (more realistically) "I wish I spent less money on luxury, so I would have had more time with people".
Also a note of caution, any time you're wasting time thats marginal time you will be begging for on your deathbed. "I wish I had one more day with my wife (or kids)" -- today could be that day, instead of wasting it, use it up fully.
That said, OP is looking for advice on how to do things, not a pat on the back for living a stress-free life, which is why I provided one.