Hypothesis

During sleep, glymphatic efflux triggers a synchronized release of brain‑derived nucleosomes into circulation, producing a transient cfDNA pulse enriched for ~150‑175 bp fragments and neuron‑specific methylation signatures. This nocturnal "autopsy" tags synapses earmarked for pruning; insufficient sleep blunts the pulse, shifting cfDNA toward longer fragments and inflammatory methylation, accelerating brain aging.

Mechanistic Rationale

• Sleep‑dependent CSF inflow expands interstitial space, increasing shear stress on perivascular astrocytes and promoting exosome shedding that carries chromatin fragments (glymphatic‑linked vesiculation).
• NMDA‑receptor mediated calcium influx during slow‑wave sleep activates calpains, which preferentially cleave linker DNA, yielding mononucleosomal cfDNA (~150 bp) from active cortical layers.
• These fragments retain methylation marks at neuronal activity‑regulated genes (e.g., BDNF, SYN1) and show low LINE‑1 methylation, distinguishing them from necrotic cfDNA released during wakefulness [https://www.fightaging.org/archives/2020/12/loss-of-line-1-methylation-in-cell-free-dna-as-a-biomarker-of-aging/].
• When sleep is fragmented, calpain activity is dysregulated, leading to incomplete digestion and enrichment of >300 bp fragments; concurrently, microglial activation raises cfDNA from inflammasome pathways, increasing methylation at NF‑κB targets.

Predictions

1. Morning plasma (within 2 h of waking) will contain a higher proportion of neuron‑derived cfDNA (estimated by deconvolution) than evening plasma, with a peak at ~160 bp fragment length [https://pmc.ncbi.nlm.nih.gov/articles/PMC11318736/].
2. The methylation fraction at a set of 12 neuron‑specific CpGs (selected from the 48‑CpG clock) will be significantly higher in morning samples, correlating positively with slow‑wave power from polysomnography [https://pmc.ncbi.nlm.nih.gov/articles/PMC11318736/].
3. Experimental sleep restriction (one night of 4 h) will attenuate the morning cfDNA pulse, increase long fragment (>300 bp) ratio, and elevate methylation at inflammation‑linked CpGs (e.g., IL6 promoter) [https://academic.oup.com/braincomms/article/7/2/fcaf125/8114922].
4. In aged mice, genetic enhancement of glymphatic flow (AQP4 over‑expression) will amplify the morning neuronal cfDNA peak and rescue age‑related methylation drift, whereas AQP4 knockout will abolish the pulse.

Experimental Design

• Recruit 30 young (20‑30 y) and 30 older (65‑75 y) participants.
• Collect paired plasma at 21:00 (pre‑sleep) and 08:00 (post‑sleep) across three consecutive nights; simultaneously record EEG to quantify slow‑wave activity.
• Isolate cfDNA, fragment size via Bioanalyzer, perform low‑coverage whole‑genome sequencing, and apply a neuronal deconvolution algorithm trained on sorted nuclei methylomes [https://pmc.ncbi.nlm.nih.gov/articles/PMC11318736/].
• Quantify methylation at the neuron‑specific CpG panel and at LINE‑1.
• Compare morning‑evening differences using paired t‑tests; test interaction with age and sleep metrics via linear mixed models.

Potential Confounds & Controls

• Peripheral leukocyte contribution controlled by leukocyte‑DNA depletion and using methylation signatures unique to neurons.
• Physical activity and food intake standardized; samples collected fasting.
• Plasma processing within 30 min to avoid ex vivo clot‑derived DNA.

Falsifiability

If morning plasma shows no enrichment of neuron‑derived cfDNA or fragment size shift relative to evening, and if sleep restriction does not alter the cfDNA metrics as predicted, the hypothesis that sleep gates a distinct nucleosomal autopsy pulse is falsified.