Study Uncovers How Mental Fatigue Causes the Brain to Zone Out - Video

A new study reveals what happens in the brain when sleep deprivation causes attention lapses-the brain flushes out cerebrospinal fluid (CSF) in large pulses, a mechanism normally seen during sleep, but intruding into wakefulness with a cost to focus. This breakthrough research, published in Nature Neuroscience, provides fresh insights into why we struggle to pay attention after a sleepless night.

Sleep deprivation is notorious for impairing mental performance, yet the brain mechanisms behind this have been unclear. The study involved 26 healthy adults aged 19 to 40 participating in well-rested and totally sleep-deprived conditions about 10 days apart.

Using simultaneous EEG caps to measure brain waves, functional MRI scans to track blood and CSF flow, and eye-tracking to capture pupil size, researchers monitored brain and body activity. Participants performed attention tasks responding to visual and auditory cues, while baseline rest data was also collected.

Findings showed that attention failures during sleep deprivation occur alongside large pulses of CSF flowing out of the brain, followed by a reverse flood when attention recovers. These CSF pulses closely mimicked patterns seen in non-REM sleep stages N1 and N2, accompanied by slow brain waves and changes in blood flow and pupil size.

The flushing out of CSF coincided exactly with moments participants “zoned out,” revealing a synchronized brain-body response. This suggests the sleep-deprived brain transiently slips into a sleep-like state without fully sleeping.

Lead researcher Laura Lewis explained that this coupling of CSF flow, brain waves, blood circulation, and pupil changes was unexpected but shows a tightly integrated system.

The study highlights the role of the autonomic nervous system in orchestrating these dynamics. While the exact functional implications remain to be explored, this work opens new avenues for understanding cognitive impairment from sleep loss and identifying potential treatments for sleep disorders.

These results underscore the biological consequences of even one night without sleep, revealing a brain desperately attempting to perform restorative “cleaning” at the cost of alertness and attention.

REFERENCE: Yang, Z., Williams, S.D., Beldzik, E. et al. Attentional failures after sleep deprivation are locked to joint neurovascular, pupil and cerebrospinal fluid flow dynamics. Nat Neurosci (2025). https://doi.org/10.1038/s41593-025-02098-8