Hypothesis During sleep, the glymphatic system actively releases brain-derived cell‑free DNA (cfDNA) fragments into the periphery. These fragments carry methylation signatures that reflect the synaptic activity and chromatin state of neurons cleared overnight. Consequently, the methylation pattern and size distribution of brain‑specific cfDNA in morning plasma serve as a readout of nocturnal glymphatic efficacy. Chronic sleep disruption impairs this efflux, altering the diurnal cfDNA epigenome and accelerating age‑related epigenetic drift.

Predictions

1. In healthy young adults, plasma concentrations of brain‑derived cfDNA (marked by neuronal methylation loci such as RBFOX3/NeuN and SNAP25) will peak 2–4 h after awakening and decline across the waking day.
2. The fragment size profile of this brain‑derived cfDNA will be enriched for mononucleosomal (~165 bp) and subnucleosomal (<100 bp) fragments during the sleep‑associated peak, reflecting nucleosome‑protected DNA from active chromatin and apoptotic fragments from cleared synapses.
3. Methylation at a set of neuron‑specific differentially methylated regions (DMRs) identified in aging studies [1] will show higher methylation (or lower, depending on the DMR) in the morning peak compared with evening trough, indicating a sleep‑linked epigenetic reset.
4. Experimental suppression of glymphatic flow (e.g., via acetazolamide or controlled sleep deprivation) will blunt the morning peak of brain‑derived cfDNA and flatten its diurnal methylation rhythm, while total cfDNA levels remain unchanged [4,5].
5. Individuals exhibiting a blunted glymphatic‑cfDNA rhythm will show accelerated accumulation of age‑related cfDNA methylation changes (e.g., global LINE‑1 hypomethylation [3]) and poorer cognitive performance over a 6‑month follow‑up.

Experimental Design

• Participants: 30 healthy adults aged 20‑35, balanced for sex.
• Protocol: Each participant undergoes two 24‑h conditions in crossover design: (a) normal sleep (8 h opportunity) and (b) total sleep deprivation. Blood drawn every 2 h across the cycle.
• Measurements:
  • Plasma cfDNA isolation, fragment size analysis by capillary electrophoresis.
  • Targeted bisulfite sequencing of neuron‑specific DMRs from [1] and repetitive elements (LINE‑1, Alu) [3].
  • Glymphatic function assessed by intrathecal contrast‑enhanced MRI (or surrogate DTI‑ALPS index) at baseline and after each condition.
  • Cognitive battery (PVT, n‑back) administered each morning.
• Analysis: Compare circadian amplitude (peak‑trough difference) of brain‑derived cfDNA concentration, fragment size ratio, and methylation beta values between sleep and deprivation conditions using mixed‑effects models. Correlate glymphatic metric with cfDNA amplitude.

Potential Outcomes and Falsifiability

• If the morning peak of brain‑derived cfDNA and its methylation rhythm are absent or inverted under sleep deprivation, the hypothesis gains support.
• If glymphatic impairment does not affect brain‑derived cfDNA dynamics, or if cfDNA patterns remain unchanged despite altered clearance, the hypothesis is falsified.
• Conversely, a persistent peak independent of glymphatic measure would suggest alternative sources (e.g., meningeal or vascular) driving the signal.

This framework directly tests whether sleep‑dependent glymphatic efflux contributes a measurable, epigenetically informative cfDNA signature to peripheral circulation, bridging the gap between nocturnal brain maintenance and daytime biomarkers of aging.