Why Your Brain 'Zones Out' When You're Exhausted — CSF Pulses, Blood Flow and Sleep‑Like Waves

Study reveals how exhaustion triggers brief 'zoning out' episodes

We've all struggled to stay focused after a poor night of sleep. A new study published in Nature Neuroscience maps what happens inside the brain during those brief attention lapses and reveals surprising links between fluid dynamics, blood flow and sleep‑like brain activity.

What the researchers did

The team recruited 26 healthy volunteers (ages 19–40; 19 female) and tested each person twice about 10 days apart: once well rested and once after sleep deprivation. In the rested condition participants slept 6.5–9 hours at home; in the deprivation condition they were kept awake for roughly 24 hours in the laboratory. On test mornings the researchers recorded brain activity with EEG, measured blood and cerebrospinal fluid (CSF) flow using simultaneous functional MRI (fMRI), and tracked pupil size with eye‑tracking.

Tasks and baseline measures

Participants performed simple visual and auditory attention tasks (press a button when they saw an image or heard a sound) and also provided 25 minutes of quiet resting data. As expected, sleep‑deprived volunteers were slower and missed more cues than when they were well rested.

Key findings

Large CSF pulses: During attention lapses in the sleep‑deprived state researchers observed surprisingly large pulses of CSF that were pushed out of the brain and then rushed back in when attention returned.

Sleep‑like slow waves: These CSF pulses coincided with slow brain waves and other signatures that typically occur during non‑REM sleep, especially transitions from stage N1 to N2.

Tightly coupled physiological changes: The CSF movement was strongly linked to pupil dynamics — inward CSF flow followed pupil dilation and outward flow followed pupil constriction — and to marked blood‑flow shifts measured by fMRI. These patterns were more pronounced after sleep deprivation.

“By measuring many different kinds of brain signals at the same time, we saw that things we thought were separate were actually moving together,” said Laura Lewis, associate professor of neuroscience at MIT and a co‑author of the study.

Interpretation and caveats

The authors propose that the sleep‑deprived brain intermittently slips into a sleep‑like state while remaining technically awake: attention lapses mark the onset of sleep‑like processes that are then interrupted before full sleep occurs. The coupling with pupil size and blood flow suggests involvement of the autonomic and circulatory systems.

However, the functional consequences of these large blood‑flow shifts are not yet clear. The team notes that future work should test whether these events influence how effectively the brain clears metabolic waste. External experts emphasized that the experiment used 24 hours of deprivation — a severe manipulation — so effects may be larger than what people experience after a typical shortened night of sleep.

Why this matters

These findings connect the subjective experience of 'zoning out' with measurable, sleep‑like brain physiology and systemic signals. Understanding these dynamics could help guide treatments for people with chronic sleep problems or conditions that impair attention.

Bottom line: When you briefly lose focus from exhaustion, your brain may be momentarily entering a sleep‑like state: CSF is flushed out and then refilled as attention returns, alongside slow brain waves, large blood‑flow changes and pupil adjustments.