The placebo effect can extend to the effect of sleep on cognitive functioning, according to a study published in the Journal of Experimental Psychology: Learning, Memory, and Cognition.

“The placebo effect is any outcome that is not attributed to a specific treatment but rather to an individual’s mindset,” the authors  explained. “This phenomenon can extend beyond its typical use in pharmaceutical drugs to improve aspects of everyday life.”
For the study, participants listened to a brief lecture about how cognitive function can be improved through better sleep. They were also informed that the kind of sleep that the brain benefits from the most is about 20 to 25 percent of a normal person’s night of sleep is REM sleep.

The participants were then told that they were wired with sensors to measure their pulse, heart rate, and REM while they slept. Then some of the participants were falsely told that they got approximately 16 percent REM sleep, while others were told they got close to 30 percent REM sleep. The researchers then gave the participants to take a test that involved adding numbers together.

Researchers found that participants who were told that they had below average sleep quality performed significantly worse on the test. Specifically, Assigned sleep quality but not self-reported sleep quality significantly predicted participants’ scores on the Paced Auditory Serial Addition Test and Controlled Oral Word Association Task.”

A follow-up experiment that included additional controls and tests confirmed the study’s initial findings. “These findings supported  the hypothesis that mindset can influence cognitive states in both positive and negative directions, suggesting a means of controlling one’s health and cognition,” the authors wrote.