If you need a nap, get a hammock. You will nod off quicker and sleep more deeply, researchers say.
The gentle rocking motion soothes us to sleep – and the effect has now been demonstrated by a study of brainwaves.
From babies cradled in their mother's arms to grandparents falling asleep in a rocking chair, it is common knowledge that rocking induces sleep. But scientists did not understand how it worked.
"It had remained a mystery," said Sophie Schwartz of the University of Geneva. She led a study involving 12 volunteers who were asked to nap on a custom-made "experimental hammock" bed that could either stay still or rock gently.
All the participants were naturally good sleepers who did not typically nap. Each took two 45-minute afternoon naps, one with the bed stationary and the other with it in motion. At the same time, their brainwaves were monitored by electroencephalogram (EEG) electrodes attached to the scalp.
"We observed a faster transition to sleep in each and every subject in the swinging condition," said co-researcher Dr Michel Muhlethaler, also from the University of Geneva.
The findings, published in the journal Current Biology, showed a "dramatic boosting" of certain types of sleep-related brainwaves associated with rocking.
Swaying from side to side specifically increased the duration of deep non-dreaming sleep, where the eyes are still, which normally accounts for about half a good night's sleep. The brainwaves also showed activity typical of deep sleep.
The next step is to see whether rocking may be useful for the treatment of insomnia and other sleep disorders, say the researchers. "Swinging" sleep might also improve memory and brain-damage repair, they say.
Children often rock themselves to sleep but the habit can be disturbing if it extends into adulthood. Called rhythmic movement disorder, it is marked by excessive rocking or banging of the head or body in bed. It is usually a response to stress. Introducing bedtime rituals to induce relaxation, such as a warm bath, can help ease the condition.
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