Sleeping in the lateral, or side position, as compared to sleeping on one’s back or stomach, may more effectively remove brain waste, and could reduce the chances of developing Alzheimer’s, Parkinson’s and other neurological diseases, according to researchers at Stony Brook University.
Stony Brook University researchers discovered this in experiments with rodents by using dynamic contrast magnetic resonance imaging (MRI) to image the brain’s glymphatic pathway, a complex system that clears wastes and other harmful chemical solutes from the brain. They also used kinetic modeling to quantify the CSF-ISF exchange rates in anesthetized rodents’ brains in lateral, prone, and supine positions.
Colleagues at the University of Rochester used fluorescence microscopy and radioactive tracers to validate the MRI data and to assess the influence of body posture on the clearance of amyloid from the brains.
Their finding is published in the Journal of Neuroscience.
Most popular position in humans and animals
“It is interesting that the lateral sleep position is already the most popular in human and most animals —even in the wild — and it appears that we have adapted the lateral sleep position to most efficiently clear our brain of the metabolic waste products that built up while we are awake,” says Maiken Nedergaard, PhD, a co-author at the University of Rochester.
“The study therefore adds further support to the concept that sleep subserves a distinct biological function of sleep and that is to ‘clean up’ the mess that accumulates while we are awake. Many types of dementia are linked to sleep disturbances, including difficulties in falling asleep. It is increasing acknowledged that these sleep disturbances may accelerate memory loss in Alzheimer’s disease.”
The brain-waste clearing system
Cerebrospinal fluid (CSF) filters through the brain and exchanges with interstitial fluid (ISF) to clear waste in the glymphatic pathway, similar to the way the body’s lymphatic system clears waste from organs. The glymphatic pathway is most efficient during sleep. Brain waste includes amyloid β (amyloid) and tau proteins, chemicals that negatively affect brain processes if they build up.
New York University Langone Medical Center was also involved in the research.
Abstract of The Effect of Body Posture on Brain Glymphatic Transport
The glymphatic pathway expedites clearance of waste, including soluble amyloidβ (Aβ) from the brain. Transport through this pathway is controlled by the brain’s arousal level because, during sleep or anesthesia, the brain’s interstitial space volume expands (compared with wakefulness), resulting in faster waste removal. Humans, as well as animals, exhibit different body postures during sleep, which may also affect waste removal. Therefore, not only the level of consciousness, but also body posture, might affect CSF–interstitial fluid (ISF) exchange efficiency. We used dynamic-contrast-enhanced MRI and kinetic modeling to quantify CSF-ISF exchange rates in anesthetized rodents” brains in supine, prone, or lateral positions. To validate the MRI data and to assess specifically the influence of body posture on clearance of Aβ, we used fluorescence microscopy and radioactive tracers, respectively. The analysis showed that glymphatic transport was most efficient in the lateral position compared with the supine or prone positions. In the prone position, in which the rat’s head was in the most upright position (mimicking posture during the awake state), transport was characterized by “retention” of the tracer, slower clearance, and more CSF efflux along larger caliber cervical vessels. The optical imaging and radiotracer studies confirmed that glymphatic transport and Aβ clearance were superior in the lateral and supine positions. We propose that the most popular sleep posture (lateral) has evolved to optimize waste removal during sleep and that posture must be considered in diagnostic imaging procedures developed in the future to assess CSF-ISF transport in humans.