Sleep and appetite may seem unrelated at first, but they have more in common than you might think. I know the last few times I was up late working, I was snacking on something. (Though, this example is complicated by the fact that stress can lead to hunger too.)

The protein leptin is associated with hunger and metabolism. When you eat, leptin is produced by fat cells. Scientists have created genetically obese animals, by removing both copies of the leptin production genes or leptin receptors. Its important to note that most obesity in humans is rarely due to this disorder (because it is a rare mutation and recessive). the discovery of leptin showed how physiological the compulsion to eat can be especially after years of bad eating habits.

During digestion, elevated levels of leptin travel up through the blood to the hypothalamus. The hypothalamus is your brain’s connection between to the endocrine (hormone) system. It plays an important role in maintaining homeostasis, circadian cycles, and controlling motivated behaviors like hunger and thirst.

When leptin levels rise they act on your hypothalamus to release neuropeptides (αMSH and CART) into the arcuate nucleus. These peptides make you feel more full and increase your metabolism. When leptin levels fall the peptides NPY, AgRP, and MCH are produced, and they act on your arcuate nucleus and lateral hypothalamic area to make you hungry and decrease your metabolism.

The hormone Ghrelin also stimulates appetite directly by acting on neurons that release NPY and AgRP.

What does this all have to do with sleep you might ask? Here it can get a little complicated, especially since Ghrelin was only discovered in 2005. Research has shown that the amount of ghrelin increases and amount of leptin decreases with a lack of sleep. Other studies (while not directly indicating ghrelin or leptin) have shown that children with even just an hour less of sleep were more likely to develop obesity.

The cause for the changes in ghrelin and leptin levels are unknown, but it could be from their involvement in maintaining circadian rhythms. Also, anatomically its makes sense that sleep and appetite are intertwined as they’re controlled in nearby areas of the brain.