Is it important to maintain a work-life balance as a PhD student? My apologies in advance for asking such a trivial question. My concerns may not be as urgent or serious as some of the others on here, but I have been worrying about this for some time now.
My life since starting my PhD has can typically be summed up as waking up early, getting to the office about 7AM and working on some part of my research until 5PM before going home to study some more theory. I adore my field - I really think I'm the luckiest person ever for having the opportunity to work on such interesting problems.
However, I often get comments about my work-life balance being unhealthy. People often tell me to get a hobby that isn't related to physics or mathematics. In fact, a friend even brought me evidence that the vast majority of highly successful scientists, including my scientific heroes, have had a creative hobby.
I've honestly tried to maintain a hobby but I keep getting pulled back into my old routine. The progression typically goes like this:

I start a hobby, say drawing.
I draw consistently for a week or two. I even buy a course book for drawing and work through it. I'm making progress, all is great.
By the third week, I'm plagued with thoughts that slowly erode my motivation to draw. Thoughts about how I could be studying complex analysis, or working on that atomic configuration generation code...
By week four I've forgotten that I was trying to draw in the first place and am now back to the old routine.

Yet, as my friends point out, the evidence really is against me here. A lot of successful scientists do have hobbies outside of their work. It strikes me that the problem might be a bit more deep-seated: it may just be that I've developed a very unhealthy work ethic that'll cause me to crash and burn down the line.
As academics, how important would you consider hobbies as a part of your lifestyle? Would you consider the cultivation of a field completely unrelated to your own to be an essential component to an academic career - or, as my eloquent colleague puts it, being human in general?
It is very important that work-life balance be available to people who need it.  Most PhD students and most scientists need some form of balance but exact needs vary.
You are not obligated to adopt someone else's idea of balance.  There is no evidence that what works for other people will work for you.
Healthy diet and exercise are strongly correlated with better health and longer lifespan.  There is very likely a causal relationship there.  But I don't think that's what you were asking.

Would you consider the cultivation of a field completely unrelated to your own to be an essential component to an academic career?

No.  Common, normal, and fair, but not necessarily essential.

being human in general?

That's more of a religious question than an academic question.