Why do we sleep? While scientists can’t provide a single answer to that question, they do know that tissue repair takes place during sleep.
Earlier this year, Vladyslav V. Vyazovskiy of the University of Surrey and Kenneth D. Harris of University College London hypothesized, in the journal Nature, that sleep gives the nerve cells in our brain downtime, so any needed repairs to them can take place and the cells don’t suffer long-term damage. Because nerve cells (neurons) affect each other, if one shuts down, others are disturbed. The only way they can rest efficiently is for many of them to shut down at once.
Now, Chiara Cirelli at the University of Wisconsin, Madison and her colleagues have found a connection between sleep and the production of myelin, the material that surround axons, long projections at the ends of nerve cells, or neurons. Electrochemical signals move along axons until they jump from one neuron to the other. This is how nerve impulses pass through the brain, spinal cord and rest of the body.
Cells known as oligodendroyctes produce myelin. They start out as oligodendrocyte precursor cells (OPCs).
Cirelli and her team knew that the neurotransmitter glutamate prevents the proliferation of OPCs.
As cells produce more glutamate during wakefulness than during sleep, the team thought the sleep-wake cycle would affect myelin production.
They studied mice, allowing some to sleep naturally, while forcing other to stay awake.
The researchers found that genes involved in promoting OPC proliferation and synthesizing myelin turned on during sleep, and that OPCs proliferated at twice the rate during sleep as they did during wakefulness. Proliferation was greatest during REM sleep.
On the other hand, expression of genes involved in cellular stress response and cell death (apoptosis) occurred in the mice kept awake.
Cirelli and her team think their research could help people with multiple sclerosis (MS) an immune disorder that causes myelin around nerve cells to deteriorate over time.
We already know that people with MS are very likely to suffer from sleep disorders, including insomnia, narcolepsy, restless leg syndrome, REM sleep behavior disorder and sleep apnea. This could create a vicious cycle, with lack of sleep preventing myelin repair. Worsening MS symptoms would then make sleep more difficult. In fact, researchers have found that sleep disturbance is more likely to cause fatigue in MS patients than the symptoms of the disease itself are.