Eight solid hours of sleep reinforces your resolve to resist sweet treats and fatty foods. But the morning after a night of tossing and turning, that chocolate-glazed donut with sprinkles in the office cafeteria looks doubly delicious, doesn’t it? Now scientists have proof that the sleepier you are, the less willpower you can muster to keep your fingers out of the cookie jar.

Researchers from Harvard Medical School tested this theory by showing 12 healthy adults photos of low- or high-calorie foods as images of their brains were scanned. The participants were all a little tired from a few hours of lost sleep the night before. They also were told they would be given a memory test afterward to make them focus on the pictures.

The volunteers also completed surveys detailing how drowsy they were during the day as well as their food preferences and usual eating habits.

For four minutes, images of healthy foods like a salad or an apple flashed before their eyes for a few seconds, alternating with visuals of more delectable dishes like a burger and fries or strawberry cheesecake. In between the food flashes, researchers snuck in shots of flowers, rocks and trees as a control.

Brain scans showed that the sleepier the volunteers felt, the less their impulse-inhibiting prefrontal cortex was activated. This means a chocolate chip cookie becomes just that much harder to pass up when you’ve skipped a few hours of sleep.

Past studies have also found that skimping on snoozing debilitates judgment — and leads to overeating.

So if you’re watching your waistline, add a few hours of sleep to your wellness regimen. With a solid seven or eight hours under your belt, you’ll be well-equipped to walk right on by the donut display. Even if they have sprinkles.