Half the reason why sleeping pills work is because of the placebo effect, according to a new review of studies in the British Medical Journal.
Researchers from Harvard Medical School, the University of Connecticut and the University of Lincoln analyzed past data from 13 studies on a type of sleeping pill called Z-drugs (also known as non-benzodiazepine hypnotics, which include the brand names of Lunesta, Ambien and Sonata). The studies included more than 4,300 people.
The researchers found "that Z-drugs did reduce the length of time it took for subjects to fall asleep, both subjectively and as measured in a sleep lab, but around half of the effect of the drug was a placebo response," study researcher Niroshan Siriwardena, a professor at the University of Lincoln, said in a statement.
Researchers said the findings have implications for people who use these sleeping pills and experience negative side effects, like problems with balance, fatigue and memory loss.
Recently, a study in the journal BMJ Open showed a link between hypnotic sleeping pill use and increased death risk. The study showed that people who take 18 or fewer sleeping pills each year have a three-and-a-half-times increased risk of death, compared with people who don't take the pills.
And the effect seems to be even higher with the more pills you take -- CBS News reported that people who take 132 or more sleeping pills a year have a five-times higher risk of death, as well as a 35 percent increased risk of developing cancer.
For more on sleeping pills -- and why we use them in the first place -- watch this HuffPost Live segment with Marc Lamont Hill and Drs. Russell Sanna and Stuart Quan, both of Harvard Medical School: