Yeah, I've noticed this, too. Not to a disturbing degree and frankly I'm more than smart enough for most of my daily activities (so I feel I can spare a few brain cells now and again) - however, yes, it's worrisome.

I've noticed that good nutrition helps this a lot.

Try to have some healthy munchies on hand... if, as you say, you can't resist junk... buy healthy junk ahead of time and be prepared.

Rice crackers + peanut butter or sunbutter are amazing.
Yogurt and fruit for the sugar cravings. Or even vitamin C chewables (in moderation).
Gatorade is better than soda, and Smart Water is better yet.

It's okay to munch out, but not on fast food and sugar. The brain needs to rebuild itself after exposure to any psychoactive.

I recommend getting plenty of DHA (one of the omega-3 fatty acids). It's great for the brain.