I think some of my bonks are psychological.
I did my first century this weekend, and at mile 85, passed by the start.
Then it was like a cloud went over. Suddenly, the road sucked, I had a headwind, too many cars, my legs felt like lead, and I got crabby. Every bump was met with "ow OW!". Every turn was "you've got to be KIDDING me!".

I ate some m&ms, raisins, lint from the bottom of my top tube bag.

I'm not sure if the lint helped any, but the rest of it did. I was back in the saddle, so to speak.

I think psychologically my eyes saw the beginning and registered it and I knew I only had fifteen more miles, but psychologically I was done.

I think sugar helps the brain, too.