When you sleep, the brain subconsciously processes a lot of information gathered during the day. This is often reflected in the fact that we dream. But besides processing an overflow of information, the accumulation of cellular waste products in our brain is happening during sleep. A mis-accumulation of these metabolic by-products plays an important role in the development of neurodegenerative diseases. For example, the accumulation and aggregation of β-amyloid proteins is hypothesized to be one major cause for Alzheimer’s disease. Generally, the body has developed its ways to eliminate toxic metabolic by-products from the system. In other parts of the body than the brain, our lymphatic system is responsible for waste removal. Since it would be fatal if any compound were able to freely diffuse between the brain and the rest of the body, the blood-brain-barrier prevents the unhindered exchange very effectively. As a consequence, it is plausible that there must be a separate “garbage truck” exclusively for the brain. This system has been identified by a group of researchers in 2013 and called the glymphatic pathway.1 In very simple terms, regulated by an expansion and contraction of the brain’s extracellular space during sleep, solutes between the incoming fluid, called the cerebrospinal fluid and the interstitial lymphatic fluid in our brain are exchanged. In this way, metabolic waste is drained from the brain.
Interestingly, the same group of researchers found in a follow-up study in rats that body posture during sleep exhibits an effect on the clearance rate of metabolic waste.2 Using different techniques including dynamic-contrast-enhanced magnetic resonance imaging (MRI) and fluorescence spectroscopy, the researchers concluded that waste removal was more efficient in the lateral position (laying on the side) compared to the prone (laying on the stomach) or supine position (laying on the back).
These findings combined may first of all explain, why sleep is essential for our survival. Second, even if there may be no simple explanation why one body posture during sleep improves the glymphatic transport compared to others, this research certainly goes in the right direction concerning fully understanding the molecular causes for neurodegenerative diseases and, thus, maybe finding a way to prevent Alzheimer’s disease.
– Kristina Klinker
1 M. Nedergaard, Science, 2013, 340, 1529–30.
2 H. Lee, L. Xie, M. Yu, H. Kang, T. Feng, R. Deane, J. Logan, M. Nedergaard and H. Benveniste, J Neurosci, 2015, 35, 11034–11044.