Similar for me. It feels like hunger, or sleep deprivation. I think it's detected by system 2, from an awareness of cognition not being great, in general. It's like a touch of delirium.

Hunger as a response to mild dehydration makes sense though. We get a significant proportion of our water intake from food. That instinct doesn't work quite as well in our engineered era of potato chips, but that's a recent problem.