Diet is important, but Make sure you're sleeping right as well. I feel like crap all the time, 'cause I've got sleep Apnea. My girl sits up and watches me sometimes, she says my snoring is really loud, and sometimes I breathe less than 4 times a minute. I'm going to a sleep specialist when I get the dough. It's hell, sometimes I'll be in bed for 12 hours and not feel like I've slept at all.
If you have a wife/gf/partner ask them to watch how you sleep some night (if they're awake). you'd be surprised to know some of the things that happen.