Food intake can affect body clocks – and vice versa. The movement of glucose and nutrients through the bloodstream to organs affects appetite, digestion and metabolism. Travelers frequently encounter stomach and digestive difficulties when crossing time zones, for example, because food intake is in conflict with the timekeeping molecules in the body’s digestive system.

“When you’re a shift worker and displace your sleep, you also displace your feeding schedule,” says sleep researcher Eve Van Cauter, a professor of medicine at the University of Chicago. The liver, pancreas and digestive system are not expecting food at the time that they’re getting it.

Sleep schedules that buck the body’s natural circadian rhythms can disrupt insulin production and other hormones that are important to weight control, Van Cauter says. In a series of studies, she found that sleep-deprived adults produce more ghrelin, a hormone that promotes hunger, and less leptin, a hormone that suppresses appetite. Thus, the brains of tired people are sending out compelling messages to eat – especially foods that are starchy, sweet and high in carbohydrates.

Accordingly, people who sleep less may have more trouble keeping their blood sugar stable. In one of Van Cauter’s studies, healthy young men were restricted to four hours of sleep per night for six consecutive nights and were found to have blood test results for insulin sensitivity so abnormal that they almost matched those of diabetics.

Other research is exploring effects of sleep and shift work on neurotransmitters called orexins. These brain chemicals make sure humans are alert when hungry to maximize food-seeking behavior.

After a big meal, fast-rising glucose levels in the body switch off orexin neurons, often making people feel sleepy – possibly an evolutionary response signaling humans to conserve energy after eating. “There is wiring in our brains that links feeding and being awake,” Van Cauter says.

Thus, eating at midnight and sleeping at noon could lay the groundwork for obesity, diabetes and heart disease.