Also, if you don’t eat when you’re hungry, it’s not simply a grumbling stomach you have to deal with. Now hunger comes with lethargy, headaches, irritability, and more. I used to be able to fast most of the day without any issue, but at some point in my mid-20s, it was like a switch got flipped. Now when my body senses low blood sugar, it basically revolts against me until I eat something.