Brain gets a washout: Why we experience brief lapses when we're sleep-deprived

Everyone who has ever stayed up all night or slept significantly too little knows that sleep deprivation leads to cognitive impairment the following day. Understanding the precise physiological reasons for this is far more difficult, because... The brain , after all, is not a muscle that gets fatigued from sleep deprivation and therefore needs to recover. Researchers at the Massachusetts Institute of Technology (MIT) in the USA have now investigated this matter using a rather elaborate experimental setup. The team, led by Dr. Zinong Yang, published their results in the journal "Nature Neuroscience" .

For the study, 26 volunteers were examined twice – once after a good night's sleep and once after a sleepless night in the laboratory. They were placed in the MRI scanner wearing an EEG cap on their heads, allowing for the simultaneous recording of an electroencephalogram (EEG) and a functional magnetic resonance imaging (fMRI) scan of the brain. In addition, the subjects' heart and respiratory rates, as well as their pupil size, were recorded using other equipment.

Equipped with these devices, the participants completed an attention test: A cross was displayed on a screen, which at irregular intervals changed into a square, at which point the subjects were instructed to press a button. In a further test, they were asked to react to an acoustic signal and again press the button. As expected, the participants performed significantly worse when they were sleep-deprived: They reacted more slowly and sometimes not at all to the signals.

The researchers observed that in situations of reduced attention, there was an increased outflow of cerebrospinal fluid (CSF) from the brain. Immediately after these lapses, the CSF flowed back into the brain. The CSF, or rather the glymphatic system it forms , is considered the brain's cleansing system, transporting metabolic waste products and other toxins to the periphery for disposal. The brain normally undergoes such active flushing processes during Sleep ; in cases of sleep deprivation, it must be made up for while awake, even though cognition suffers as a result, according to the researchers' assumption.

Another observation was that several physiological changes occurred in a coordinated manner. For example, the subjects' pupils constricted approximately 12 seconds before a CSF flush and dilated again afterward. Simultaneously, their heart and respiratory rates also decreased. This suggests that all these events are controlled by a higher-level system, explains senior author Professor Dr. Laura D. Lewis in a statement from MIT . This control may be exerted by the sympathetic nervous system via the neurotransmitter norepinephrine.