Hangry

Though most frequently mentioned as a joke or a jab at someone's food-seeking tendencies, the concept of being "hangry" might, in fact, be the outcome of the activities a collection of brain cells that serve a clear evolutionary purpose.

Agouti-related protein (sometimes called agouti-related peptide), or AgRP, is produced in the hypothalamus, which among other things is the part of our brain that regulates subconscious systems like our mood, heart rate, and body temperature.

It also regulates our sense of hunger, and AgRP co-expresses (shows up alongside) neuropeptide Y (or NPY) in a slew of cells that help us maintain healthy levels of energy throughout our bodies.

One of the ways it does this is by increasing our appetite, which (alongside decreasing our metabolism and thus reducing how much energy we expend) helps us continue to function without shutting down (or harvesting our organs for sustenance) when we're low on nutritional fuel.

So when we're running out of easy-to-access sources of energy, these peptides flip a flurry of internal switches that make us feel hungry, while also slowing things down a bit so that our bodies become more efficient, using less energy until we can top-up the tank.

Until we're able to do this, our blood sugar is reduced, and the hormone adrenaline is released throughout our bodies to help us kick our glucose (which we use for fuel) back up to normal levels—but this also has the effect of triggering something akin to our fight-or-flight-or-freeze response.