You'll Perform Better If You Think You Slept Well Sleep is subject to the placebo effect: study By Matt Cantor, Newser User Posted Jan 26, 2014 9:42 AM CST 7 comments Comments Sleep quality is subject to the placebo effect. (Shutterstock) (Newser) – Got a big day ahead? You're better off thinking you slept well, even if you didn't, a study suggests. Researchers told subjects a normal night of sleep consisted of about 20% to 25% REM sleep. Less than 20%, the subjects heard, meant a worse performance on cognitive tests, while more than 25% meant scoring well on such tests, the Telegraph reports. Researchers duped these subjects into believing that sensors attached to their bodies were tracking their sleep quality. After a night's sleep, some subjects were told they spent 16.2% of the night in REM sleep, while others were informed they'd spent 28.7% of the night in REM. That was all made up, the Smithsonian reports. Next, the subjects took tests involving adding numbers. Subjects who'd been told they'd slept poorly did worse on the tests, reports blogger Eric Horowitz. Those who heard they slept well did better, the Independent reports. In other words, the subjects experienced a "placebo sleep" effect. Indeed, the effect isn't limited to pills: We can get placebo exercise, too, the Smithsonian notes.