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May 17, 2021

Does Sleep Flush Wastes From the Brain?

Author Affiliations
  • 1Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts
JAMA. 2021;325(21):2153-2155. doi:10.1001/jama.2021.5631

Why do humans sleep? Most people spend one-third of their lives asleep. For most of human history, individuals have been much more vulnerable to enemies and predators when sleeping. But still, everyone sleeps and cannot help but do it. So, there must be a good reason for sleep, an evolutionary advantage.

One reason for sleeping may be to rest the brain and body. Nevertheless, most organs continue to work during sleep. In particular, the brain is highly active during sleep.1 Sigmund Freud thought one purpose of sleep was to grapple with negative thoughts buried in the unconscious through dreams. Sleep definitely helps to consolidate memories and learning. Some have speculated that during sleep unused synapses are pruned, strengthening the rest of the synapses in the same way that pruning dead branches enhances the health of a rose bush. Together, these all might seem reason enough to sleep.

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    5 Comments for this article
    Our Sentient, Cognitive, Human Capability
    Paul Nelson, MS, MD | Family Health Care, PC, retired
    This essay is a real gem. It broadens understanding of the fragile biology of our brain. I might add that it's possible that sleep also represents a biological "defragger" for our interconnected short-term and long-term memory functions, especially in conjunction with the REM cycle.
    CSF Clearance During Sleep
    Charles Brill, MD - Retired | Thomas Jefferson University
    1 - I didn't see mention of arachnoid granulations as a pathway for getting CSF from the subarachnoid space into the venous sinuses.

    2 - Does brain atrophy, commonly seen in seniors, widen the glymphatic system, potentially allowing more CSF clearance?
    Science Catches Up With Art
    Wm Prendergast, MD | Retired; Clin. Asst. Prof. Ophthalmology, OR Hlth Sci. U.
    “Sleep that knits up the raveled sleave of care,
    The death of each day's life, sore labor's bath,
    Balm of hurt minds, great nature's second course,
    Chief nourisher in life's feast.”

    William Shakespeare; Macbeth
    Brain as Kidney
    William Aldis, M.D. | Faculty of Public Health, Thammasat University, Bangkok
    I see an analogy between the waste elimination of the brain described here and renal function. We can compare (for example) "glymphatic filtration rate" to glomerular filtration rate. The anatomic structures are different obviously, but the physiological processes are similar. I wonder if much of what is known about renal function and renal failure could be applied to this context, with some useful results.
    Arachnoid Granulations
    Charles Brill, MD - Retired | Thomas Jefferson University
    What happened to arachnoid granulations? What is now considered their function in CSF absorption?