We know babies benefit from being rocked to sleep - now a study suggests it helps adults sleep better too.
Researchers from the University of Geneva built a special bed that rocked gently throughout the night.
They tested it on 18 young adults and found they woke up fewer times and slept more deeply than on a normal bed.
Scientists said the rocking motion resulted in a longer period of slow brainwaves which caused deep sleep, and improved their memory.
The volunteers spent three nights at a sleep laboratory in Geneva: one to get them used to sleeping there, one on a rocking bed and the other on the same bed, but in a still position.
Electrodes recorded their brainwaves, and found that the period of deep sleep was extended by rocking.
Laurence Bayer, lead study author and researcher at UNIGE Faculty of Medicine, pointed out that the rocking motion was slow enough not to cause nausea.
"A hammock would probably not be as efficient, although people often report a sense of relaxation when rocked in a hammock," she said.
"In our paper we test the effect of rocking on one night, but we have no idea if the effects will still be there over a long-term period."
The researchers also found the adults had better memory recall in the morning if they slept on the rocking bed.
Aurore Perrault, another researcher at the faculty, said: "To see if this also affected memory, we subjected our participants to memory tests: they had to learn pairs of random words in the evening and remember them in the morning when they woke up," she said.
"Here too, rocking proved beneficial: the test results were much better after a night in motion than after a still night."
Rocking mice sleep more quickly
A parallel study conducted by the University of Lausanne in Switzerland looked at the effect of gently rocking the cages of mice while they went to sleep.
It found that rocking reduced the time they needed to fall asleep and increased their sleep time.
However, it did not appear to increase sleep quality, like it did in human beings.
The study in mice also proved that the vestibular system, the system that tells the brain when our head is moving, has to be functioning for rocking to be effective.
Researchers agreed that in the absence of rocking beds being widely available, more research was needed to determine how the findings could help people with sleep disorders.
Prof Sophie Schwartz, a neuroscientist at UNIGE, and author of the study, said the research explained why people fell asleep on trains, and other vehicles.
"I was contacted by someone in America who works on a high crane, which moves gently all day," she said. "He told me that now he understands why he sleeps so deeply during his after-lunch nap."