One of the strangest findings to emerge from the world of obesity science lately is that people who sleep less tend to weigh more. But until recently, we have been stifling our yawns and scratching our heads about why: Does lack of sleep alter our biology? Or does it affect our eating behavior? Now two brain-imaging reports suggest the answer is both.

The first study, published in March in the Journal of Clinical Endocrinology & Metabolism, looked at the effects of one night of no sleep. The second, published in April in the American Journal of Clinical Nutrition, tested the impact of nearly a week of more commonly experienced levels of sleep deprivation (four hours of sleep for six nights).

Both studies used functional MRI to measure brain activation as their subjects viewed food pictures—analogous to being bombarded with a stream of McMuffin ads after a long night of working (or partying). Each study discovered that sleep loss caused areas within a key motivation network, including the striatum and anterior cingulate cortex, to go into overdrive at the mere sight of food. The same circuit perks up when addicts view images of their substance of choice.

“Calories are energy, and your brain subconsciously knows they will wake you up,” says Marie-Pierre St-Onge of Columbia University, lead investigator of the April study. She likens the superresponsive sleep-poor brain to that of someone who has lost weight on a drastic diet—devouring the first snack you can get your hands on is a “no-brainer.”

Scientists do not fully understand how sleep loss affects the machinery of neural motivation. Past studies have established that the stress of sleep deprivation puts the autonomic nervous system on alert, leading to increases in the hunger hormone ghrelin and decreases in the satiety hormone leptin. These changes may be detected by the brain's motivation circuits—which respond by keeping an eye out for doughnuts.

Christian Benedict, a neuroscientist at Uppsala University in Sweden who co-led the March study, is also exploring whether sleep restriction could interfere with the way our brain perceives the taste of high-calorie foods.

Whatever the underlying biology, it seems that skimping on sleep could well make us hungry as well as irritable. So if you're watching your waistline and feeling snoozy, it's probably wise to avoid the breakfast buffet until you get a chance to nap.