Hypnoses may prove even more effective than drugs in helping some people achieve restful "slow-wave" sleep, a new study suggests.

The study, published in the peer-reviewed scientific journal Sleep, describes how listening to hypnotic audio designed to help induce slow-wave sleep (SWS) can make some people up to 80 percent more likely achieve this restful state of sleep, compared to trying to sleep without aid.

Researchers from the Psychological Institute of the University of Zurich and the University of Fribourg  in Switzerland and the University of Tübingen in Germany determined this after combining data on several identical sleep studies. These studies had a total of 70 young women come to one of a number of sleep laboratories to take a 90 minute nap during midday.

According to the study, none of these women reported having a history of mental illness sleep difficulty, or widespread pain (a symptom of restless sleep) prior to the study.

Right before lying down, some women were asked to listen to a hypnosis tape designed to encourage SWS, while other women were asked to listen to a recording of non-hypnotic spoken word that served as a control.

While these women slept, researchers measured their sleep patterns with an electroencephalogram (EEG). According to the study, these EEG scans primarily focused on identifying how long each participant experienced SWS - a restful state of sleep characterized by stable and markedly slow electrical brain activity.

Interestingly, the researchers found that many of the women who listened to the SWS tape were up to 80 percent more likely to experience prolonged states of SWS.

"Hypnotic suggestions might be a successful tool with a lower risk of adverse side effects than pharmacological treatments," the team concluded.

However, don't toss away your sleeping pills just yet. Before starting the study, the researchers also assessed each participant on their natural hypnotic suggestibility in accordance with the Harvard Group Scale of Hypnotic Susceptibility.

Women who proved highly suggestible showed the greatest sleep improvement with the hypnoses, but women who were less susceptible showed no improvement at all.

It also should be noted that this study only tested a small sample of women. Many neurologists theorize that the brains of men and women can hugely vary in patterns and activity, especially following puberty.

The study was published in the June issue of Sleep.