Why hammocks are best for afternoon naps

It's all in the same swaying motion you use to put fussing babies to sleep.

GENEVA - The mystery of why it is virtually impossible to lie in a gently rocking hammock without falling asleep has been solved.

An analysis of the brainwaves of sleeping subjects revealed that a slow swinging motion both helps us fall asleep faster and slip into a deeper sleep than in a stationary bed.

The findings also help to explain the age-old belief that cradling a baby will send it off to sleep, and why it is so difficult to stay awake in a rocking chair.

The study, published in the journal Current Biology, found that lying on a slowly rocking bed helped each subject in a small test group fall asleep more easily, and spend longer in a deeper stage of sleep.

The Swiss scientists who carried out the research suggested that the swinging motion's effect could help synchronise our brain activity into a pattern associated with sleep.

One of the study's co-authors, Dr Sophie Schwartz of the University of Geneva, said: "It is a common belief that rocking induces sleep. We irresistibly fall asleep in a rocking chair and, since immemorial times, we cradle our babies to sleep.

"Yet, how this works had remained a mystery. The goal of our study was twofold: To test whether rocking does indeed soothe sleep, and to understand how this might work at the brain level."

Twelve volunteers were asked to take an afternoon nap on a custom-made bed or "experimental hammock" that could either remain still or be made to rock gently.

During two 45-minute naps, one with the bed stationary and one with it swaying, their brain activity was monitored. The swaying motion of the bed was found to increase the duration of N2 sleep, a form of non-rapid eye movement sleep that normally accounts for about half of a good night's rest.

While in the swaying bed participants also recorded more slow oscillations in the brain and flurries of activity known as sleep spindles, which correspond with a deeper level of sleep.

Dr Michael Muhlethaler, another co-author, said: "We observed a faster transition to sleep in each and every subject in the swinging condition, a result that supports the intuitive notion of facilitation of sleep associated with this procedure.

"Surprisingly, we also observed a dramatic boosting of certain types of sleep-related (brainwave) oscillations."

The researchers said the results could help them find potential treatments for sleeping disorders such as insomnia. THE DAILY TELEGRAPH