An overtired brain increasingly breaks down synapses and permanently activates leukocytes — a characteristic typical of neurodegenerative diseases.
Italian scientists have discovered a mechanism explaining why humans with chronic sleep deprivation are more likely to develop dementia.
The brain uses sleep as a time for reorganization, breaking down any synapses that are rarely used. If it is not given this chance because it is kept awake for too long on a regular basis, this process is accelerated, breaking down more than double the number of synapses than when it is well rested
This acceleration also increases the number of synapse residues in the head, which stimulates leukocytes to dispose of these residues. Patients with Alzheimer’s disease have also exhibited a slightly elevated level of leukocyte activity. Consequently, the results of the research conducted by Marche Polytechnic University indicate a connection between sustained sleep deprivation and brain deterioration.
Frantic brain care
The brain consists of two types of cell: neurons and neuroglial cells. The small neuroglial cells provide support for the larger neurons that we use to learn and think. The team of scientists used two types of neuroglial cell in order to examine how this type of cell reacts to sleep deprivation: astrocytes, which break down the weak synapses between neurons, and immunological microglia, which clear the residues of these synapses.
The scientists compared three groups of test participants. The first group slept normally, the second group was kept awake for eight hours, and the third group was not allowed to sleep for five successive days in order to simulate the effects of chronic sleep deprivation. Brains scans were then taken of each test subject, which revealed that astrocytes were active in 6 percent of the synapses of those who slept normally; for those who were tired, this number jumped to 8 percent, and for those who were overtired for a sustained period, 13.5 percent. This means that the brains of those who were subjected to constantly disturbed sleep were destroying a massive number of neural connections.
“Our research is the first to show that some synapses are literally eaten up by astrocytes if the brain is deprived of sleep,” says Research Director Michele Bellesi. This is not necessarily a bad thing, as this “purge” was particularly prevalent around largest synapses, which were more developed and used more intensively. “These synapses are like old furniture; they require a lot more attention and care.”
A constant strain on the immune system
However, Bellesi was concerned about the increased level of microglia cell activity he observed in those participants who were subjected to chronic sleep disturbances. Such increased activity is usually a characteristic of neurodegenerative diseases such as Alzheimer’s, Parkinson’s or multiple sclerosis: The overstimulated microglia were also poisoning the surrounding neurons with their high-strength antibodies.
A link between sleep deprivation and brain diseases has already been observed in the glymphatic system in the form of meningeal lymphatic vessels, which also contain neuroglial cells. The aqueous liquids in this system “flush” the brain — especially during sleep. The new study has opened up the possibility that overactive microglia cells overload the lymphatic vessels with an increased amount of cell residues, meaning that these residues can no longer be efficiently removed and therefore accumulate in this system. Further tests are needed to understand exactly how microglia and the glymphatic system interact when the body is deprived of sleep.