rest and digest turkey

If you ate a big Thanksgiving dinner yesterday, you probably felt drowsy and sluggish afterward, a phenomenon often referred to as a “food coma”. The belief that it’s caused by the tryptophan in turkey is a long busted myth, and in fact it can happen after any carb-heavy meal. The reasons for this post-food slump are relatively well understood from experiments in humans and other mammals. For one, big meals trigger a “rest and digest” response when the food reaches the stomach and small intestine (via activation of the parasympathetic nervous system). This diverts energy to digestion, making you feel sluggish and sleepy.  But wait, there’s more! Eating a meal full of carbs and other sugars also stimulates the release of insulin, which triggers a process to take up nutrients from the bloodstream. But a side effect of this process is an increase in the amount of serotonin and melatonin in the brain, two chemicals that are associated with drowsiness (and often happiness).

So having a full stomach makes you sleepy, but did you know that the relationship between sleep and food goes deeper than that? For example, studies in animals from fruit flies all the way to humans have shown that having an empty stomach can keep you awake. Even worse, chronic sleep deprivation stimulates appetite and can lead to weight gain. Studies in humans have uncovered strong correlations between sleep disorders like insomnia and obesity-related disorders such as diabetes and cardiovascular disease. One of the reasons for this is that sleep loss wreaks havoc on the levels of certain hormones, such as the “hunger hormone” ghrelin.

Thus, even though sleeping and eating are mutually exclusive behaviors (you can’t sleep while you’re eating), they’re also obviously connected. Both are essential for survival, so the brain may often be promoting at least one of them. But how does the brain decide which behavior is more important at any given time? When you’re hungry, it seems more important to stay awake and find food to prevent starvation, but sleep deprivation also has health consequences, so the brain needs to ensure you get enough sleep.

So how are sleep and hunger connected in the brain? Are they independent, so that hunger suppresses sleep, and sleepiness stimulates hunger? Or maybe they arise from the same mechanism, which signals in turn for either sleep or eating depending on your body’s needs. Answering this question will improve our understanding of sleep- and obesity-related disorders and how they’re linked, a necessary step before treatments can be developed.

Fruit flies make a great animal model for answering detailed questions like these, which take place on a molecular and cellular scale. Although much research has yet to be done, there have already been some informative findings. For starters, scientists have found that insulin-producing cells in the fly brain regulate both feeding and sleep, providing a cellular link between these two behaviors (remember that insulin’s activities are partially responsible for the food coma after Thanksgiving dinner). Their findings suggest that these cells integrate information from other brain regions about sleep need and hunger, and may even potentially act as a “behavioral switch” to signal which behavior is more important at any given time.

Other fruit fly researchers have found a link between eating and sleeping in a single molecule called “NPY-like short neuropeptide F” (sNPF). sNPF had already been shown to regulate food intake, but recently it was found that sNPF also promotes sleep. sNPF is similar to a mammalian chemical called “neuropeptide Y” (NPY), which also plays a little-understood role in sleep and eating in mammals. NPY has recently been studied as a possible drug target for obesity treatment in humans even though its role in other behaviors is unclear, so a better understanding of this chemical is essential to account for possible side effects such as sleep disruption.

Research in a variety of animal models (and humans) has shown us that sleep and hunger are related. Overall, the combined results of fruit fly research suggests that sleep and feeding behaviors may arise from shared mechanisms in the brain. Future studies will pin down the exact mechanisms and apply those findings to more complex mammalian systems. Hopefully, this research will lead to the development of drugs that can help with sleep- and obesity-related disorders. In the meantime, I think I’ll try drinking coffee with Thanksgiving dinner.

Happy Thanksgiving!