A new study from Brown University has found that rewards improve performance on a visual perceptual task only if participants sleep after training.

The new findings may have particular implications for students tempted to sacrifice sleep in favor of late-night study sessions, said study corresponding author Yuka Sasaki, a professor of cognitive, linguistic and psychological sciences at Brown University.

“College students work very hard, and they sometimes shorten their sleep,” Sasaki said. “But they need to sleep to retain their learning.”

The study found that prefrontal, reward-processing areas became active only during REM sleep

The study asked young adults to identify a letter and the orientation of a set of lines on a busy background. One group was told to refrain from eating or drinking in the hours leading up to the task. During the test, they were given drops of water as a reward for correct responses.

Compared to groups not rewarded during training, participants exhibited significant performance gains, but only if they slept after the training session.

The researchers believe that reward, or anticipation of it, reinforces neural circuits between reward and visual areas of the brain. And these circuits are then more likely to rebound during sleep and support learning. EEGs confirmed increased brain activity in the prefrontal, reward-processing area and decreased activation in the untrained visual areas of the brain.

The study found that prefrontal, reward-processing areas became active only during REM sleep. REM sleep appears to be particularly important for task learning — likely because connections are reorganized and optimized during this sleep stage — and it may be linked to the activation of reward-processing areas of the brain. Consistent with this theory, the rewarded study participants exhibited longer periods of REM sleep compared to those who did not receive a reward during training.

Sasaki added that physical-based rewards, like food and water, may have a stronger impact on neural circuits compared to rewards such as money.

“Water deprivation may be fundamental,” she said. “When you’re really thirsty and you get water as a reward, the impact of that reward may be more prevailing to the brain.”

In addition to Sasaki, other Brown University authors on the study were Masako Tamaki, Aaron V. Berard, Tyler Barnes-Diana, Jesse Siegel and Takeo Watanabe.