Blood cells enter the brain during sleep to haul away lipids, a fly study finds

The dominant modern account of what sleep is for is a cleaning account: during sleep, the brain disposes of things that accumulate during waking. Almost every version of that account is brain-centric. The clearance is done by the brain, to the brain, within the skull.

A paper published in Nature on February 11 reports something outside that frame. In fruit flies, cells from the circulation — not from the brain — travel to the brain during sleep and remove lipids that built up while the animal was awake [s1].

What the researchers found

The cells in question are haemocytes: macrophage-like immune cells in the Drosophila circulation. The authors report that haemocytes track to the brain during sleep and take up lipids that have accumulated in cortex glia as a result of wake-associated oxidative damage [s1].

To test whether this mattered, they screened phagocytic receptors expressed in haemocytes — the receptors these cells use to recognise and engulf material. Knocking down one of them, a member of the Nimrod receptor family called eater, reduced sleep [s1].

That is the load-bearing result, and it runs in an unexpected direction. If haemocyte clearance were merely something that happens to occur during sleep, disabling it should not change sleep itself. Instead, breaking the clearance machinery shortened sleep — which is more consistent with the clearance being part of what sleep is doing than with it being a passive by-product.

The downstream damage

Loss of eater disrupted both haemocyte localisation to the brain and lipid uptake, and the consequences propagated [s1]:

• Brain levels of acetyl-CoA rose, along with acetylated proteins — including the mitochondrial proteins PGC1α and DRP1
• Mitochondria became dysregulated, showing high oxidation and reduced NAD⁺
• Memory was impaired and lifespan was reduced

The chain the authors describe runs from a failure to clear lipids, through a shift in the brain's acetylation state, into mitochondrial dysfunction, and out to two of the coarsest measures of an animal's function: whether it can remember and how long it lives.

The claim about mammals, and its status

The authors suggest that these peripheral blood cells are precursors of mammalian microglia — the brain's own resident immune cells [s1]. If that lineage relationship holds, a function first identified in flies has at least a plausible route into mammalian biology.

That is a proposal in the paper's closing sentence, not a demonstration. Nothing in this work shows peripheral immune cells entering a mammalian brain during sleep, and the paper does not claim otherwise.

Why the fly result is not trivial anyway

What is novel here is the location of the mechanism. Most sleep-clearance work over the past decade has concentrated on fluid movement within the brain — the glymphatic literature — and on glial function. This paper puts a circulating cell type in the causal chain, and connects it to mitochondrial state through protein acetylation rather than through bulk fluid flow.

The oxidative-damage framing is also worth noting: the lipids being cleared are described as accumulating in cortex glia specifically as a consequence of wake-associated oxidative damage [s1]. That makes waking the source of the load and sleep the disposal, which is the shape the clearance hypothesis has always predicted but has rarely been able to demonstrate with a specific substrate and a specific carrier.

What this does not say

It does not say that human sleep clears brain lipids by this route. It does not say anything about neurodegenerative disease in people, about how much sleep anyone needs, or about what happens when humans sleep badly. It is one paper, in an invertebrate, describing a mechanism that has not been shown to operate in any mammal.

What to watch

Whether the same phagocytic-receptor dependency can be demonstrated for microglial lipid handling in a mammalian model, and whether the acetyl-CoA and NAD⁺ signature — which is measurable — appears in mammals after sleep deprivation.

This article describes basic research in an animal model and is not medical advice.

Sources

• [s1] Cho B, Youngstrom DE, Killiany S, et al. Sleep-dependent clearance of brain lipids by peripheral blood cells. Nature, published online 2026-02-11.