In the study, the researchers kept 18 volunteers awake in the lab for a lengthy 22 hours. Then they put these very tired individuals in a dark, warm fMRI machine and told them not to fall asleep. Of course, their eyes did close periodically—and the scanner was able to pick up what happened in their brains when they did. If their eyes closed, the participants were quickly urged to open them again by recorded audio messages. To make sure the results didn’t simply come from the eyes closing, the researchers compared these sleepy scans to those of well-rested people just closing their eyes.