Why Hammocks Make Sleep Easier, Deeper

I wonder if this has anything to do with the idea that increased sleep quality via construction of nightly nests has been critical for the evolution of advanced cognitive capabilities in great apes (including humans). Seems like a slowly swaying hammock could be a lot like a tree bough - MA

From NPR
by NANCY SHUTE

Napping in a hammock is one of the more delightful tasks of summer, and Swiss researchers say they now know why.

The gentle rocking motion makes people fall asleep faster, and they sleep deeper. Those changes in brain activity may inspire new ways to help insomniacs, the researchers say.

Neuroscientists at the University of Geneva rigged up a bed so it would sway gently from side to side every four seconds, considerably slower than the pendulum on a cuckoo clock. "This rocking is very gentle, very smooth, oscillating every four seconds," Sophie Schwartz, a professor of neurology who led the study, told Shots. "It's not like rocking like you would see some mothers rocking their babies, it's more gentle."

A dozen adult research subjects napped on the bed for 45 minutes while scalp electrodes recorded brain activity. During one nap the bed swayed; for another, it was stationary.

The scientists weren't too surprised to find that people fell asleep faster when the bed rocked. But they were surprised at the big difference that rocking made in brain activity.

Rocking increased the length of N2 sleep, a form of non-REM sleep that takes up about half of a good night's rest. It also increased slow oscillations and "sleep spindles." Sleep spindles are brief bursts of brain activity, which look like sudden up-and-down scribbles on an electroencephalogram.

"We were basically trying to find a scientific demonstration of this notion of rocking to sleep," Michel Muehlethaler, a professor of neuroscience who conducted the research with Schwartz, tells Shots. The fact that the brain waves changed so much, he says, was "totally unexpected." The results were published in the journal Current Biology.

Sleep spindles are associated with tranquil sleep in noisy environments and may be a sign that the brain is trying to calm sleepers stuck in them. Spindles also have been linked with the ability to remember new information. And that is associated with the brain's ability to rewire itself, known as brain plasticity.

That ability is important in recovery from stroke, and the researchers say that rocking while sleeping should be tested on people with strokes or other brain injuries. Rocking is "changing things in your brain," Schwartz says.

The Swiss scientists are eager to try the rocking bed on night-time sleepers, to see if it might help with insomnia and other common sleep disorders. But Shots readers may not want to wait for those results, and instead head directly to the back yard and their own time-tested research tool, the hammock.

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Citation
Bayer L, Constantinescu I,Perrig S, Vienne J, Vida P-P, Mühlethaler M, Schwartz S (2011) Rocking synchronizes brain waves during a short nap. Current Biology 21(12) R461-R462 doi:10.1016/j.cub.2011.05.012

Abstract
Why do we cradle babies or irresistibly fall asleep in a hammock? Although such simple behaviors are common across cultures and generations, the nature of the link between rocking and sleep is poorly understood [1,2]. Here we aimed to demonstrate that swinging can modulate physiological parameters of human sleep. To this end, we chose to study sleep during an afternoon nap using polysomnography and EEG spectral analyses. We show that lying on a slowly rocking bed (0.25 Hz) facilitates the transition from waking to sleep, and increases the duration of stage N2 sleep. Rocking also induces a sustained boosting of slow oscillations and spindle activity. It is proposed that sensory stimulation associated with a swinging motion exerts a synchronizing action in the brain that reinforces endogenous sleep rhythms. These results thus provide scientific support to the traditional belief that rocking can soothe our sleep.