People in many high stress professions - such as medicine, law enforcement or the military - often have to make quick decisions when they're not well rested. New research confirms what common sense has long held: people don't do their best thinking when they're sleep-deprived.

Major William Killgore is a U S Army psychologist at the Walter Reed Army Research Institute who studies sleep. He says when a person makes an emotional or moral decision, it activates a part of the brain, just behind the eyes, called the medial pre-frontal cortex. "That part of the brain is really important for the ability to make good emotional decisions," he explains. "So when people are making those highly emotionally-charged, moral choices, that part of the brain lights up. But when you're making a less emotionally-charged decision, that part of the brain is less active."

Killgore's prior research used imaging scans to look at the brains of sleep-deprived people as they made routine decisions. As expected, the scans showed the pre-frontal cortices of these people were less active. But the longer people went without sleep, the more that area seemed to shut down. So Killgore wondered if sleep-deprivation also impaired moral decision-making.

He tested the idea by keeping several dozen volunteers up for three days. Then he began asking them to solve a set of moral dilemmas, such as whether to let one person die to save many others. "What we found is that when people are making highly emotional decisions, those that are involving the moral personal dilemmas, they took much longer when they were sleep-deprived than they did when they were at baseline." Killgore's team measured the volunteers' responses at two different time points: when they were well-rested and after 53 hours without sleep.

Some of the subjects had been given caffeine to help them stay awake, but stimulants didn't seem to make a difference in their decision-making. Killgore says after three days, volunteers made poor choices, and took longer to make them.

Killgore says this research serves to reinforce the importance of adequate sleep. He points out, "We live in a society where people are going longer [without sleep] than they should because we have bright lights and TVs and lots of other things going on all of the time and so people aren't getting enough sleep. And just because you feel like you're functioning well doesn't mean that all aspects of your thinking are functioning well."

He stresses the importance, even in everyday life, of not making major decisions, especially those that involve emotions, when you're sleep-deprived. His research appears in the journal Sleep.