Hypothesis

Just as the brain’s glymphatic system performs an active triage of neural architectures during sleep, the bone marrow possesses a sleep‑gated interstitial clearance mechanism that removes aged hematopoietic stem and progenitor cells (HSPCs) to maintain niche homeostasis. Chronic sleep disruption impairs this marrow ‘autopsy,’ leading to accumulation of dysfunctional HSPCs, myeloid skewing, and reduced erythropoietic output in aging.

Mechanistic Model

1. Sleep‑regulated interstitial fluid (ISF) flow in marrow sinusoids – During NREM sleep, heightened parasympathetic tone and reduced sympathetic outflow lower arteriolar resistance, augmenting marrow ISF velocity. This flow is mediated by aquaporin‑4 (AQP4)‑like channels expressed on sinusoidal endothelial cells and perivascular stromal cells, analogous to CNS glymphatic conduits.
2. Circadian gating via Per2 in stromal niches – Stromal Per2 expression peaks during the rest phase, driving rhythmic secretion of CXCL12 and SCF that create transient low‑adhesion windows. In these windows, ISF flow facilitates detachment and transport of senescent HSPCs toward macrophage-rich endosteal zones.
3. Phagocytic clearance by niche macrophages – Bone‑marrow macrophages express elevated MerTK and CD163 during sleep, recognizing phosphatidylserine‑exposed, aged HSPCs. Efferocytosis triggers anti‑inflammatory TGF‑β release, suppressing IL‑6/TNF‑α that otherwise inhibit erythroid differentiation.
4. Feedback to erythropoiesis – Efficient removal of aged HSPCs renews niche space for youthful, erythroid‑biased progenitors, enhancing EPO sensitivity and burst‑forming unit‑erythroid (BFU‑E) output.

Predictions & Experimental Tests

• Prediction 1: In aged mice, pharmacological enhancement of marrow ISF flow (e.g., intrathecal AQP4 agonist) during sleep will increase the flux of CD150⁺CD48⁻ HSCs toward endosteal macrophages and raise BFU‑E colonies by ≥30% versus vehicle.
• Prediction 2: Genetic deletion of Aqp4 in leptin receptor⁺ stromal cells will abolish the sleep‑dependent rise in marrow ISF tracer (e.g., Evans‑blue‑albumin) accumulation in macrophages and cause nocturnal accumulation of annexin‑V⁺ HSPCs, exacerbating anemia after chronic sleep fragmentation.
• Prediction 3: Administering a MerTK blocking antibody at the onset of sleep will inhibit senescent HSPC clearance without affecting total HSC numbers, leading to elevated marrow IL‑6/TNF‑α and reduced erythroid recovery after sleep deprivation.
• Falsifiable outcome: If enhancing ISF flow fails to improve erythropoiesis or if blocking macrophage phagocytosis does not worsen anemia under sleep disruption, the hypothesis that marrow interstitial clearance drives a nightly hematopoietic autopsy is refuted.

Potential Confounds & Controls

• Control for systemic corticosterone levels by adrenalectomy or metyrapone to isolate local marrow effects.
• Verify that observed changes are not secondary to altered locomotion or feeding by using pair‑fed, activity‑matched cohorts.
• Use flow cytometry with mitochondrial‑membrane potential dyes (e.g., TMRE) to distinguish truly senescent HSPCs from quiescent subsets.

By linking sleep‑driven marrow fluid dynamics to the selective removal of aged hematopoietic elements, this hypothesis extends the brain ‘autopsy’ metaphor to a tangible, testable mechanism linking sleep health to age‑related anemia.