By Emma Young
As every parent knows, gentle rocking helps a baby to fall asleep. Now a new study, published in Current Biology by researchers in Switzerland, shows that a rocking bed also benefits adults, extending the time that they spend in deep, slow-wave sleep, helping them sleep more soundly, and increasing their memory consolidation through the night. A related rocking study on mice, conducted by a team involving some of the same researchers, and published in the same journal issue, helps to reveal how rocking might have these effects.
Laurence Bayer at the University of Geneva and colleagues had previously found that continuous rocking during a 45-minute nap helped adults to fall asleep and to sleep more deeply. For the new study, they wanted to explore the effects of rocking during an entire night.
Eighteen healthy young adults spent a first night getting used to the custom-built bed, which swayed back and forth every four seconds, moving a total of 10.5 centimetres. Next came two experimental nights. One was spent in the rocking bed, and one in a standard bed. During both nights, EEG was used to monitor their brain activity.
Rocking didn’t help the participants enter the first stage of early, light sleep any faster, nor was their REM sleep affected. But they did spend less time in the first few stages of sleep, and nearly a quarter more time in deep slow-wave sleep.
Bayer and her colleagues also noticed that rocking increased the number of “sleep spindles” during slow wave sleep. These bursts of coherent brain activity are known to help prevent a sleeper from briefly waking – and on the nights the participants were rocked, there was an association between an increase in the “fast” form of sleep spindles and reduced awakenings. (Though brief arousals are common during sleep, they do interfere with normal cycling between the various sleep stages.)
Spindle activity during slow wave sleep is also known to relate to the replaying of memories, which is important for memory consolidation. On the evening before each of the experimental nights, the participants were given a series of word pairs to learn before being tested immediately and then again when they woke in the morning. They performed better on the morning memory test after a night of rocking than after a night in a normal bed. In fact, after a rocking night they did even better on the test than they had the previous evening (this overnight improvement didn’t occur after a night in a standard bed). “This increase in overnight memory accuracy was supported by a decrease in the number of errors and an increase in the number of correct responses only during the rocking night,” the researchers note.
How might rocking increase slow-wave sleep duration and reduce disruptive waking? “While the exact underlying neuronal mechanism cannot be fully elucidated here, we hypothesize that the rocking effect would be driven by the vestibular system,” they write.
Evidence for this comes in part from the companion study on mice. All-night rocking had slightly different impacts on mice vs. people: when their cages were rocked, the mice fell asleep faster and stayed asleep for longer, but there was no evidence that they slept more deeply. The effects of rocking vanished, however, in mice bred to lack functional otolithic organs. These organs, which form part of the vestibular system, sense forward/backward and sideways motion.
The researchers think rhythmic rocking may exert a kind of synchronising action in the brain, inducing a rhythmic pattern of activity in neural pathways from the thalamus, which receives signals from the vestibular system, on into the cortex, directly influencing the appearance of fast sleep spindles.
More work is clearly needed to establish exactly what’s going on. But as far as the observed sleep effects themselves are concerned, as the researchers conclude, “These results may be relevant for the development of non-pharmacological therapies for patients with insomnia or mood disorders or even for ageing populations, who frequently suffer from decreased sleep and/or from memory impairments.”
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