Participants were hooked up to equipment that they were told would read their pulse, heart rate, and brainwave frequency, though it actually just measured their brainwave frequency. They were told that these measurements would allow the researchers to tell how much REM sleep they’d gotten the night before. This was not true.
Participants who were told they had above-average REM sleep performed better on the test, and those who were told their REM sleep was below average performed worse, even when researchers controlled for the subjects’ self-reported sleep quality.
Fascinating. Simply believing that you got a good night’s sleep helps you perform better, even if you didn’t actually sleep as well as you thought.