You always suspected as much, and it turns out you were right: while writing all those late-night term papers in college, you really couldn't resist that pizza slice (or pie). And now there's proof as to why: two recent studies suggest that most people are more likely to succumb to unhealthy foods when they don't get enough sleep.

Both scientific studies—one at the New York Obesity Research Center, the other at the University of California, Berkeley— used similar methodologies: they allowed some participants to sleep a full eight hours or more, and deprived others. The participants were then hooked up to a brain scanner and asked to choose between junk food (donuts, pizza) and fruits and vegetables. For sleep-deprived participants, the MRI images showed "impaired activity in areas of the brain associated with complex decision-making," meaning it was harder to decide to eat fruits and vegetables, and easier to succumb to the salty, carby foods.

When tired, "the brain can no longer convince itself that a healthy food is the right choice due to health benefits, and instead focuses on taste," says Susan E. Matthews at My Health News Daily. It's all "relative to cognitive control," says St-Onge. "Your guard is somewhat down when you're tired." Even when you know you shouldn't order an extra slice of pizza, "when you're tired you might just decide to go for it." And evolution may play a role. When you're fatigued, "your body would want calorie-dense foods that give you quick energy," says dietician Samantha Heller. "In an evolutionary sense," eating such foods would give you a short-term competitive advantage.

Does this all sound familiar? I know that I'm still particularly susceptible after a long trip - I haven't slept well, I'm tired, and it's easier to get takeout! What about you?

Read More: Why Sleepy People Love Pizza, at The Week

Related: The 10 Foods That Most Affect Weight Gain and Loss

(Image: Stephanie Barlow)