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Overnight, immune cells dive into marrow
Cells head to marrow
The evidence that immune cells dive into the bone marrow at night is pretty solid. Neutrophils come out of the marrow and settle into tissues, where they seem to deposit collagen around the body. Other cells go quiet in the tissues and leave them alone for repair.
Fluid and puffiness
Sleep also moves fluid. Bags under the eyes after a bad night are lymph pooling, fluid that has not been cleared, and a couple of good nights make them go away. Eyes get glassy for the same lymphatic reason.
A nightly cleanup
Plumbing alone does not explain why one lousy night leaves a person catching the cough from across the room. Immune cells in the eye clear the lens, and cleanup of that kind works best when nothing new is making a mess, like washing a windshield when no flies are hitting it. The body uses a lot of ATP through the day and leaves byproducts behind, and sleep is when production stops and cleanup can get ahead.
Why do immune cells follow a daily schedule?
Immune cells carry their own molecular clocks, and the body's central clock also sends them timing cues through hormones such as cortisol. Studies in mice have found that the numbers of circulating white blood cells rise and fall over the day, partly because the cells' exits from bone marrow follow a rhythm. Mice are nocturnal, so their peaks fall at different clock times than in humans. The same timing logic is now being studied in how vaccines and immune drugs perform at different hours.
How does sleep clear waste from the brain?
In 2012, Maiken Nedergaard's lab described a system in mice called the glymphatic system, in which cerebrospinal fluid flows along channels around brain blood vessels and flushes out waste. A 2013 study in Science from the same lab found that the space between brain cells grew by about 60 percent in sleeping mice. That wider space let fluid wash through faster. Among the waste products cleared was beta-amyloid, the protein that builds up in Alzheimer's disease.
Why do neutrophils have to be replaced so often?
Neutrophils are the most common white blood cells and the first to arrive at an infection. They survive only hours to a couple of days once released from the bone marrow. The marrow therefore makes on the order of a hundred billion of them each day. Their short life limits how much damage their toxic contents can do to healthy tissue.
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