Got a big day ahead? You're better off thinking you slept well, even if you didn't. Researchers told subjects a normal night of sleep consisted of about 20% to 25% REM sleep. Less than 20%, the test subjects heard, meant a worse performance on cognitive tests, while more than 25% meant scoring well on such tests. Researchers duped their test 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.

... Then the test subjects took tests involving adding numbers. Subjects who'd been told they'd slept poorly did worse on the tests. Those who heard they slept well did better. In other words, the subjects experienced a "placebo sleep" effect.