For each experiment, the women were outfitted with electrodes to monitor brain-wave activity and sleep patterns.
While lying in a bed with the lights off, the women were exposed to varying audio tapes of about 13 minutes in length. Some provided a hypnotic suggestion to sleep deeper, while others were designed to be neutral in content.
The women were allowed to fall asleep during or after the audio feeds, and all were woken up after they had spent a total of 90 minutes napping.
Hypnosis did not improve sleep in those deemed low-suggestible to hypnosis, the study found. However, women in the highly suggestible group slept 67 percent more and saw their "deep sleep" time rise by roughly 80 percent following exposure to audio hypnosis.
Other phases of sleep did not appear to be affected by hypnotic suggestion. However, the team further observed that slow-wave activity during the deep sleep phase was "significantly enhanced" following hypnosis. This suggests that not only does hypnosis boost deep sleep quantity, it may also improve deep sleep quality.
The team acknowledged that the study only included female participants. This was by design because men have a tendency to be less suggestible to hypnosis overall. However, men who are highly suggestible would probably derive similar sleep benefits from hypnosis, Rasch's team said.
And given that roughly half the general population is believed to be moderately suggestible to hypnosis, the team concluded that hypnosis could ultimately prove to be a very useful -- and side-effect free -- way to help improve sleep.
"I have to emphasize that we did not focus on sleep-disorder patients," said Rasch. "These were all healthy people. So while our findings are really promising, we do not yet have proof that hypnosis will help people who suffer from sleep disturbances. I would say it would. But it's not yet proven," he added.
"Also, although the impact of hypnosis on suggestible people was really clear and, I would say, amazing, I do not think that hypnosis would ever completely replace the need for sleep medication for those who need it," Rasch said. "It could certainly reduce the need. But I don't expect miracles from hypnosis. It's a technique to consider. But in really strong cases of sleep disturbance a medical intervention might be necessary."