Inability to pay attention when sleep-deprived could be brain wanting to clear waste: Study

Attention lapses due to sleep-deprivation could be because the brain is trying to catch up with waste cleansing — which typically happens while sleeping — during the day, a new study has suggested.

The cerebrospinal fluid, which flows into and around the brain and spinal cord cushions the central nervous system from shocks, helps remove waste from the brain that was built during the day through 'flushing'. It is considered important for maintaining brain health and function.

"If you don't sleep, the cerebrospinal fluid waves start to intrude into wakefulness where normally you wouldn't see them. However, they come with an attentional tradeoff, where attention fails during the moments that you have this wave of fluid flow," said Laura Lewis, associate professor of electrical engineering and computer science at the Massachusetts Institute of Technology and senior author of the study published in the journal Nature Neuroscience.

Sleep is critical for normal daily functioning, with deprivation known to cause an impaired cognitive function, such as in remembering and paying attention, among other deficits. Problems in the brain's waste clearance has been suggested in previous studies to lead to dementia.

The study tested 26 participants for their ability to pay attention — once following a night of sleep deprivation in the lab, and once when they were well-rested — in a visual and an auditory task.

For the visual task, the participants were asked to press a button when a cross on a screen in front of them would change to a square. For the auditory one, they were asked to do the same when they heard a beep.

Recordings of brain activity using an electroencephalogram and functional magnetic resonance imaging (fMRI) scanner revealed that sleep-deprived participants performed worse, compared to the well-rested ones.

Time taken to respond to changes in visuals and audio was more and in some cases, the participants did not register the change — an attention lapse, the researchers said.

Among the physiological changes seen during an attention lapse, the researchers noted that the cerebrospinal fluid flowed out of the brain during the lapses and flowed back in after each lapse.

The result suggests that "at the moment that attention fails, this fluid is actually being expelled outward away from the brain. And when attention recovers, it's drawn back in," said Lewis.

The researchers proposed that when the brain is sleep-deprived, it begins to compensate for the loss in cleansing that normally occurs during sleep, possibly at the cost of attention.

"One way to think about those events is because your brain is so in need of sleep, it tries its best to enter into a sleep-like state to restore some cognitive functions," lead author Zinong Yang, a postdoctoral associate at MIT, said.

A reduced breathing and heart rate and narrowing of pupils were among the other physiological changes related with an attention lapse, pointing to how effects of sleep-deprivation are body-wide, the researchers said.

They "demonstrate that attentional failures during wakefulness after sleep deprivation are tightly orchestrated in a series of brain-body changes, including neuronal shifts, pupil constriction and cerebrospinal fluid (CSF) flow pulsations, pointing to a coupled system of fluid dynamics and neuromodulatory state."