Science

How rocking the bed promotes deep sleep and improves memory

Rocking allows the brain to slowly fall in synchrony with the rocking stimulation and speed, promoting deep NERM sleep

Soja Rajakumari,sojaa was a beautiful lullaby sung by K. L. Saigal; M.L Vasanthakumari has sung a series of lullabies in Tamil. Both are available on the Web. Lullabies are slow, gentle and accompany the rocking speed of the cradle in which the baby is placed. Most families invariably have a cradle, be it made of sandalwood and swung by silk ropes (chandan ka palna, resham ki dori, sung by Hemant Kumar) or a poor mother’s make-do one using a saree hung from a tree — it is the rocking that puts the baby to sleep. The lullaby is to soothe the baby’s mood. That music soothes the brain is known and is being understood in neurobiological terms (e.g., Chanda and Levitin, ‘The neurochemistry of music’, Trends in Cognitive Sciences, April 2013). Thus, in the cradle the music is a bonus.

That rocking is sleep-inducing even in adults, particularly senior citizens who sit in a rocking chair trying to read a book after lunch. What is it about rocking that promotes sleep? This question is being answered recently by a Swiss neurobiology group. They have published two papers in the January 2019 issue of Current Biology (http://doi.org/10.1016/j.cub.2018.12.007, and also https://doi.org/10.1016/j.cub.2018.12.028). The group decided to recruit 18 young adults as volunteers, put in small metal electrode discs on their scalps and recorded the activity of their brains using a computer device called an electroencephalogram or EEG. Each volunteer was put on a bed, and the bed was rocking rhythmically the whole night at a slow speed for 8 hours. For comparison, they were again asked to sleep later on the same beds, but this time with no rocking and their EEG recorded. All other conditions of the environment were the same, so that they could study the effect of rocking on the sensory processing of the brain. Comparison of the brain waves revealed that rocking promoted the volunteers falling asleep sooner, sleep more deeply and wake up less frequently.

Stages of sleep

Sleep in humans occurs in two different steps. During sleep, our eyes actually move even when closed. One phase is when the eyes more rapidly (referred to as rapid eye movement or REM sleep) and the other where REM does not occur or NREM. Most dreams occur during REM sleep and it is thought to play a role in memory, mood and learning. During rocking, the EEG pattern showed slow oscillations. Rocking allowed the brain to slowly fall in synchrony with the rocking stimulation and speed, thus entraining the brain and promoting deep NERM sleep.

How does this entrainment or synchrony occur? The brain has a central part called the thalamo-cortical network. This is a system of neural fibres that pass on electrical signals in the brain connecting the thalamus and the cortex regions and integrates the sensory information into so that the brain “feels”; it also plays a role in memory.

Does rocking during sleep affect memory? It does seem to help. In order to test this, the group gave the volunteers a simple test of pairing two words appropriately. The test was given after they had a night’s sleep in the rocking bed and again after they slept on the same bed which was not allowed to rock. The volunteers did better in the morning test when they were rocked! Thus the group concludes that rocking boosts deep sleep, sleep maintenance and memory in healthy sleepers. How does this translate to rocking babies in cradles? Or should one test a set of volunteer senior citizens napping on a rocking chair post-lunch, and again when they nap when the chair does not rock? Interesting!

When you rock

The group wanted to know whether rocking promotes sleep in other mammals too. They decided to use mice as experimental animals. They placed electrodes on their heads and monitored the EEG (and also electromyograph patterns or EMG, which record how the skeletal muscles are affected) signals while the mice were sleeping in rocking cages and in stationary cages. Mice needed a faster rocking rate than us humans, but otherwise behaved the same way as humans do.

It has been believed that the effects of rocking are mediated through the vestibular system in the head. The vestibular system starts from the inner ear and goes to the central part of the brain. It is what helps us keeping balance while walking, feel the movement of an elevator going up or down, and lose balance just upon hearing a deafening noise (as on hearing Deepavali crackers). The role of the vestibular system in the mouse experiments become clear when the scientists tried the experiments using mice that have vestibular deficiency; these animals were insensitive to rocking. Based on these experiments with mice and men, the authors extrapolate to how they may be relevant to people with insomnia, mood disorder and memory impairment. They suggest that it might help such people, and even ageing populations, if they use rocking chairs while napping, and if possible, try and obtain a gently rocking bed.

If lullaby soothes the baby, should senior citizens too have any kind of soothing tunes as they nap? One site — sleepadsvisor.org — suggests a list of best relaxing music for sleeping, and that listening to music that has relaxing tones is best. Classical music pieces that have repeating slow tempo and no high notes are good, and meditation music and nature sounds are excellent. The group named Indian Meditation Music gives several examples, most of them with flute as the basic instrument. Another site called Yellow Brick Cinema offers “Relaxing Piano Music 24/7”. And the Youtube also offers “Raag Hansdhwani-Relaxation Music Therapy” and a whole hour of “Tampura Key D1 hour for Hindustani and Carnatic Relaxation Music”. Take your pick, relax and enjoy a nap!

dbala@lvpei.org

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Printable version | May 26, 2020 4:24:15 AM | https://www.thehindu.com/sci-tech/science/how-rocking-the-bed-promotes-deep-sleep-and-improves-memory/article26161611.ece

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