When Wakefulness Starts to Slip: Brain Fog, Sleep Deprivation, and the Brain’s Fluid Pulses

You know this feeling. After a night of little or no sleep, the next day begins normally enough. You sit down at your desk, open your laptop, and start reading an email. Halfway through, you realize you’ve absorbed none of it. You scroll back up and try again. Later, someone asks you a simple question - something you know like the back of your hand - and there’s a brief, uncomfortable pause before the answer forms. It’s subtle. You’re awake and functioning, but your mind feels slightly delayed, just out of sync with what’s happening around you.

We tend to call this brain fog. It doesn’t feel dramatic or alarming. It feels diffuse. Reaction times stretch. Small details slip through. There are brief gaps: moments where attention flickers, even though you’re clearly awake. And because you’re awake, it’s easy to assume that this is just mental fatigue, something you should be able to override with effort.

But what if those small attentional failures are not simply lapses in focus? What if they reflect a coordinated shift in your brain’s internal state - involving not only neural activity, but the systems that regulate blood flow and even the fluid that surrounds it?

The brain’s fluid system in motion: CSF

The brain's state is shaped not only by its electrical activity, but by its physical environment in which that activity unfolds. Your brain floats in cerebrospinal fluid, or CSF - a clear liquid that cushions neural tissue and helps maintain stable conditions within the rigid space of your skull. Inside that enclosed space, brain tissue, blood, and fluid share volume. When blood vessels expand and blood volume increases, CSF is pushed outward. When blood volume decreases, CSF flows back inward. This back-and-forth movement happens continuously and is driven by changes in blood flow.

What makes this important is that blood flow itself changes with brain state. During non-REM sleep, the brain produces large, slow waves of electrical activity. These are accompanied by equally slow changes in blood oxygen levels. This means that as your blood volume rises and falls in these waves, CSF moves in pronounced pulses. In this state, neural activity, blood flow, and fluid movement become tightly coordinated. Fluid movement, then, is not just random background motion: it reflects your brain’s physiological state. When neural activity and blood flow shift, CSF follows.

If these systems shift together, can that coordination be observed in the living human brain? The answer is yes. With fast functional MRI, researchers can track large-scale blood-oxygen fluctuations and the associated movement of cerebrospinal fluid - even while you’re still awake.

That raises a deeper question. If non-REM sleep is defined by tightly coordinated neural, vascular, and fluid rhythms, what happens when sleep is removed - but wakefulness continues?

From Missed Moments to Moving Waves: How Sleep Loss Triggers a Coordinated Brain–Body Shift

To answer this question, researchers studied 26 participants who each came into the lab twice: once after a normal night of sleep and once after staying awake all night under supervision. This allowed the researchers to compare the same people in a well-rested and sleep-deprived state.

The following morning, participants lay awake while researchers recorded their brain activity using EEG and fast fMRI at the same time. Pupil size, heart rate, and breathing were also monitored. Importantly, the goal was to examine attentional lapses during confirmed wakefulness - not during sleep.

While being scanned, participants performed up to four runs of a sustained attention task called the Psychomotor Vigilance Test. In this task, a light appears at unpredictable intervals, and participants press a button as quickly as possible. The researchers examined both very slow responses (>500 ms) and missed responses. These missed responses - called omissions - are widely used markers of attentional lapses and allowed the researchers to pinpoint the exact moments when attention failed.

As expected, sleep deprivation slowed reaction times and increased omissions. But the central question was what happened in the brain at the exact moment attention failed. Because brain activity, blood flow, and CSF were recorded simultaneously, the researchers could align all of them to each attentional lapse. Using fast fMRI, they were able to measure both large-scale blood-oxygen fluctuations and CSF inflow in the fourth ventricle of the brain.

What they observed was not a simple drop in focus. Instead, attentional lapses were accompanied by a coordinated cascade of changes:

• Brain activity linked to alert wakefulness decreased
• Slower, sleep-like activity increased
• The pupil briefly became smaller
• Heart rate and breathing shifted
• A large, low-frequency wave of blood flow moved across the brain
• At the moment a response was missed, CSF was pushed outward from the brain’s central fluid spaces. A few seconds later, the flow reversed and moved back inward.

Crucially, all of this happened while participants remained awake with their eyes open. These were not episodes of full sleep. They were brief state transitions lasting only a few seconds.

When the researchers looked more closely, they found something even more striking. They separated these flickers in attention into two categories: moments when attention first began to fail, and moments when attention returned. This pattern moved in opposite directions. When attention broke down, brain activity linked to alertness dropped and fluid moved outward. When attention came back, brain activity increased again and the fluid moved inward. In other words, the brain’s electrical signals, blood flow, and fluid movement shifted one way as attention declined - and then shifted back as attention recovered.

That reversal is important. It suggests these were not random mistakes. They were coordinated changes in brain state. This is why the findings matter. Brain fog after sleep deprivation may feel like simple tiredness, but these lapses coincide with measurable changes across brain activity, blood flow, bodily arousal, and fluid movement - even when you’re fully awake.

Why Brain Fog Is a Sign of Lost Alignment

Think back to that moment at your desk - rereading the same line, searching for a word that should come easily. It feels minor. But the study shows that in the sleep-deprived brain, those moments coincide with coordinated shifts in brain activity, blood flow, and the movement of fluid around the brain. What feels like a lapse in concentration is actually your brain briefly changing how its systems are working together.

Staying awake and focused depends on those systems moving in sync. When sleep is withheld, that synchrony becomes fragile. Patterns normally seen during sleep - slow, widespread fluctuations in brain and blood activity - begin to appear during wakefulness at the precise moments attention falters. These are not random slips. They are brief shifts in how the brain is operating.

That’s why this finding matters. Sleep is not just time spent offline. It is the condition in which neural, vascular, and fluid rhythms are organized and stabilized. When that stabilizing influence is reduced, instability does not stay hidden - it becomes measurable, and it becomes noticeable.

Brain fog, then, is not merely tiredness. It’s the moment you feel that coordination begin to slip.

Reference

Yang, Z., Williams, S. D., Beldzik, E., Anakwe, S., Schimmelpfennig, E., & Lewis, L. D. (2025). Attentional failures after sleep deprivation are locked to joint neurovascular, pupil and Cerebrospinal Fluid Flow Dynamics. Nature Neuroscience, 28(12), 2526–2536. https://doi.org/10.1038/s41593-025-02098-8

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