Decoded: Why we crave junk food after sleepless night?

Researchers have figured out why people crave more calorie-dense, high-fat foods such as doughnuts, chocolate chip cookies, and potato chips after a sleepless night, and how to help thwart those unhealthy choices. According to the study, published in the journal eLife, our nose -- or olfactory system -- is affected in two ways by sleep deprivation. First, it goes into hyperdrive, sharpening the food odours for the brain so it can better differentiate between food and non-food odours. Then there is a breakdown in the communication with other brain areas that receive food signals, and with that our decisions about what to eat change, the researchers said.

"When you're sleep-deprived, these brain areas may not be getting enough information, and you're overcompensating by choosing food with a richer energy signal," said Thorsten Kahnt, an assistant professor at Northwestern University Feinberg School of Medicine in the US. "But it may also be that these other areas fail to keep tabs on the sharpened signals in the olfactory cortex. That could also lead to choosing doughnuts and potato chips," Kahnt added.

Past research shows sleep deprivation increases certain endocannabinoids, which are naturally produced by the body and are important for feeding behaviour and how the brain responds to odours, including food smells. "We put all this together and asked if changes in food intake after sleep deprivation are related to how the brain responds to food odours, and whether this is due to changes in endocannabinoids," Kahnt said.

The researchers involved 29 men and women, ages 18 to 40 in a two-part experiment. Study participants were divided into two groups. One got a normal night's sleep, then four weeks later, were only allowed to sleep for four hours. The experience was reversed for the second group. The day after each night -- good sleep and deprived sleep -- scientists served participants a controlled menu for breakfast, lunch and dinner, but then also offered them a buffet of snacks.

Scientists measured how much and what they ate. "We found participants changed their food choices. After being sleep deprived, they ate food with higher energy density like doughnuts, chocolate chip cookies and potato chips," Kahnt said. Researchers also measured the participants' blood levels of two endocannabinoid compounds -- 2AG and 2OG. One of the compounds, 2-OG, was elevated after the night of sleep deprivation and this increase was related to changes in food selection.

In addition, scientists put subjects in an fMRI scanner before the buffet. They then presented them with a number of different food odours and non-food control odours while they observed the piriform cortex, the first cortical brain region that receives input from the nose. They observed that activity in the piriform cortex differed more between food and non-food odours when subjects were sleep-deprived.

"When the piriform cortex does not properly communicate with the insula, then people start eating more energy-dense food," Kahnt said. Other than getting more sleep, it may help to pay closer attention to how our nose sways our food choices, researchers said. "Our findings suggest that sleep deprivation makes our brain more susceptible to enticing food smells, so maybe it might be worth taking a detour to avoid your local doughnut shop next time you catch a 6 am flight," Kahnt said.