You’re probably aware of the placebo effect—taking a pill of any sort can make a person feel better, even if that pill has no active ingredients. But it turns out that the placebo effect doesn't just work with gel caps. It's possible to get placebo exercise and even placebo sleep.

In this study, researchers gave participants a short lecture about how getting more and better sleep improves cognitive function. They also told them that during the normal person’s night of sleep, about 20 to 25 percent of it is REM sleep, the kind that helps the brain most. The researchers then attached sensors to the subjects and told them the sensors would measure pulse, heart rate and REM while the subjects slept. (This was a lie.) Some of the subjects were later told that they got 16.2 percent REM sleep, while the others were told they got 28.7 percent REM sleep. (This was also a lie.)

The researchers then gave their subjects a test called the Paced Auditory Serial Addition Test. The test is simple enough: it asks people to add a bunch of numbers together. Eric Horowitz at the blog "Peer-reviewed by my neurons" explains the results:

Draganich and Erdal found that participants who were told they had below average sleep quality performed significantly worse on the PASAT. At the same time, self-reported sleep quality was unrelated to PASAT performance. A follow-up experiment that included additional controls and three other cognitive tests largely confirmed the initial findings. In addition, the performance of participants on a verbal fluency test called the COWAT showed that not only does telling people they had below average sleep quality lead to inferior performance, telling them they had above average sleep can lead to superior performance.

Basically, if you tell somebody that a "treatment"—a pill, better sleep, more exercise—is going to work, it will probably work. And this has proven true time and again, for all sorts of things beyond straight medication. A study that told a group of hotel housekeepers that their work was good exercise found that those workers scored higher on health indicators than a group of hotel housekeepers who didn’t think they were getting exercise on the job. People who are told that athletes have good vision have better vision when doing athletic activities. And simply thinking you got better sleep makes your brain work better.

Scientists are still trying to figure out how to give people actually better sleep.