[This is part of a series on the tools I used to write my PhD. Check out the other parts here.]
Intense focus on a particular mental challenge, problem or project has the tendency -- at least in my experience -- to become an all-or-nothing proposition. Any non-PhD-related activities are considered unimportant or irrelevant, and you end up sitting in front of your chair for hours on end.
I've already written about the importance of periodic breaks in your work routine. These breaks were short breaks that I was referring to, but you also need to find a way to include -- your own situation permitting, of course -- ample opportunity for recharging your physical body and needs.
This is common sense. We all know that we should probably sleep more and move more. Most of us aren't getting enough of either, and we feel its effects on our concentration or we feel the physical aches and pains in your body that come after a few hours sitting hunched over in a chair in front of a laptop.
If you're doing intense work thinking about particular problems, getting more sleep and movement will really invigorate your ability to keep doing that. Your body will thank you and you will feel the difference in your work and attention.
Movement doesn't need to be something as structured as going to work out, or a specific activity, even. The mental and physical benefits of long walks (or multiple shorter walks over a single day) are pretty well established in the scientific record, I think, and I know that when I make sure to include lots of walking in my day I generally feel better. (I actually have a bunch of quantitative data to back that up from various tracking projects that I maintain, but that's a topic for another day).
All of this is not about being prescriptive, but I think you'll find that if you can find a way to sleep a little more and move a little more each day, your body and mind (and your PhD) will thank you. This is all about realigning your own sense of what you want for yourself with the reality of how you go about your day.