Sleep 'cleanses' the brain of the day's waste and regenerates us

It may sound like a well-constructed alibi, fully in tune with the seasonality of the saying 'April, sweet sleep'. But no. As of today, there is scientific proof that sleep does not only lead to 'catching no fish', but that it actually represents a fundamental activity (and this is not an oxymoron) for 'cleaning up' the brain. This is demonstrated by a discovery made by a group of Finnish researchers at the University of Oulu (Finland), which is somewhat reminiscent of what happens in large cities at night, when the traffic calms down and leaves the field to the cleaning vehicles, free to spring into action and flood the streets and pavements with water. Mutatis mutandis, the same thing, in a more measured and elegant way, also happens in our central nervous system, which takes advantage of the hours of sleep to get rid of the waste accumulated during the day. A hitherto invisible process that it has finally been possible to observe in real time and in a non-invasive manner. This is thanks to researchers in Oulu who have developed a technique capable of monitoring changes in the flow of cerebral fluid.

Our brain, like other organs, continuously produces metabolic waste, waste substances, by-products of neuronal activity that, if not properly disposed of, can accumulate and become potentially harmful.

Taking care of this particular 'waste disposal' is a system based on the movement of liquids (in particular cerebrospinal fluid or CSF) through the brain tissue. This flow is induced by three types of natural pulsations of our organism: cardiac pulsations, generated by the beating of the heart, respiratory pulsations, linked to the rhythm of breathing, and finally the waves produced by the walls of the blood vessels. During sleep, these pulsations change in a surprising and somewhat paradoxical way. Thanks to a new ultra-fast magnetic resonance imaging (the examination takes five minutes), developed by the Functional Neuroimaging Research Group (OFNI), the Finnish researchers were able to observe the movement of water molecules in the human brain in real time. In this way, they discovered that during sleep, the cleansing system goes into 'turbo mode'. Respiratory and vasomotor pulsations accelerate, favouring the flow of fluids; only cardiac pulsations slow down, possibly due to the reduction in blood pressure values from vasodilation.

From this play of flows, an extremely efficient filtration results, a kind of 'self-washing' for the brain that 'flushes' itself with greater intensity. But of course, what happens during sleep is not just a matter of hydraulics. What also changes is the way the brain coordinates its activities. Whereas during daytime wakefulness the process involves first activating neurons and then causing an increase in blood flow, during sleep this relationship is no longer unidirectional and this hierarchy can change. Slow vasomotor waves (below 0.1 Hertz) begin to locally influence not only fluid movement, but also the electrical activity of the brain. In short, it is no longer just neurons that dictate the activity agenda: this complex fluid traffic is also coming into play, which can modulate brain activity. An effect that is particularly evident in the posterior areas of the brain, such as the sensory cortex; here, a marked increase in fluid flow through the brain tissue is observed, signifying a greater removal of waste.

Understanding how the brain manages to clean itself is not just a biological curiosity, but a crucial health issue. As we age, this system slows down, with the risk of metabolic waste accumulating. A phenomenon known to be associated with memory disorders and various neurodegenerative diseases.