If you didn’t sleep so well last night, get ready to loosen your belt a notch. Multiple studies have shown that losing sleep significantly increases the desire for high-calorie foods and makes people eat more. But a new study gives a surprising explanation for why getting less shuteye makes us fatter.

The study, published in the journal Nature Communications, suggests that being sleep deprived both increases brain signals motivating us to eat fattening foods, and decreases our ability to resist temptation. In the study, subjects’ food choices were measured after they slept a full night (about eight hours) in a lab, and then again after skipping a night of sleep. They were fed snacks to make up for the extra energy they spent staying up. In the morning, their brain activity was monitored while they rated how desirable they found different pictures of foods. To sweeten the deal, they were given whichever food they rated the highest.

Interestingly, the subjects reported no difference in how hungry they were regardless of whether they had slept or not. But those who had skipped a night’s sleep rated high-calorie, fattening foods as much more appealing compared to other healthier options. In fact, after being sleep deprived, people wanted on average 600 calories more. And the sleepier they felt, the more likely they were to gravitate towards high-calorie foods.

The researchers discovered that sleep deprivation increased brain signals in the amygdala, which responds to food stimuli and participates in the motivation to eat. At the same time, brain signals decreased in the frontal cortex, which is partially responsible for appetite control and decision making. Losing sleep, therefore, acts like a double whammy by making people both crave fattier foods and have more trouble resisting them.

Past studies have supported a strong link between sleeping and eating habits. Large population studies have shown that both adults and children who get less sleep are more likely to be overweight or obese. A study earlier this year showed that even five days of suboptimal sleep is enough to make people pack on nearly two pounds. But prior researchers had thought that weight gain from decreased sleep was a result of people needing to eat more to stay awake. Now you can blame those high-calorie cravings on your head, not your stomach.