HI friend,

I have similar conditions. This feeling always comes and goes.
last year I used to go swimming early morning before starting my study. However, now I think it was not a good idea. it took my morning time and made me tired for the rest of the day. now I changed my exercise time to evenings or nights. It works much better for me.

I work 6 days and rest on Sundays. Believe me! I still feel guilty on Sundays! HA!
It's 3 weeks that I am sick after a long time of hard working and making myself exhausted.
Don't push yourself to do PhD works beyond your natural abilities. A PhD student needs to rest, exercise and have good food to be survived..

Good luck!