The later you hit the sack, the more likely you are to grab some Jack in the Crack in the morning.

That's because lack of sleep compromises high-level brain functioning, according to a new study published August 6 in Nature Communications. The research found that sleep deprivation leads to brain changes that make it harder to make good decisions and easier to give in to cravings.

“What we have discovered is that high-level brain regions required for complex judgments and decisions become blunted by a lack of sleep, while more primal brain structures that control motivation and desire are amplified,” study senior author Matthew Walker, a UC Berkeley professor of psychology and neuroscience, said in a press release.

Because of the brain's altered state, high-calorie foods are more appealing, Walker said.

The researchers measured brain activity in 23 healthy young adults after a normal night's sleep and again after a sleepless night. They took fMRI scans as the subjects looked at 80 different pictures of food that ranged from high- to low-calorie, and healthy to junk food, such as hamburgers, pizza, doughnuts, strawberries, apples and carrots.

The subjects were asked to rate how much they wanted to eat each particular food. As a little reward, they were given the item they wanted the most after the experiment (this also works with dogs).

The scientists found that lack of sleep negatively affected activity in the brain's frontal lobe, which is the region in charge of decision-making processes. Sleepiness also increased activity in the deeper centers of the brain that are involved in reward pathways.

When junk foods such as pizza were offered to sleep-deprived subjects along with leafy vegetables and whole grains, they eagerly chose the greasier path. However, subjects were better able to make healthier choices after adequate rest.

Previous studies have linked poor sleep to greater appetites, particularly for sweet and salty foods, but the latest findings provide a specific brain mechanism explaining why food choices change for the worse following a sleepless night, Walker said. The choices appear to have less to do with the body's energy needs than with impaired impulse control.

“These results shed light on how the brain becomes impaired by sleep deprivation, leading to the selection of more unhealthy foods and, ultimately, higher rates of obesity,” lead author Stephanie Greer, a doctoral student in Walker's Sleep and Neuroimaging Laboratory, said in the press release.

On a positive note, Walker said, the findings indicate that “getting enough sleep is one factor that can help promote weight control by priming the brain mechanisms governing appropriate food choices.”

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