The concept of “sleep debt”, or the difference between the amount of sleep someone needs, and the amount of sleep the person actually gets, is something a lot of people are familiar with. However, the question of “how exactly does the brain track and calculate this sleep debt?” was one that remained a mystery. However, recent research by Mark Wu, Professor of Neurology at John Hopkins University (Maryland), and his colleagues, may have found the answer.
In his research, Wu discovered neurons present in mice’s brains, which tracked how much they slept, and then helped them recover from their sleep debt. The next step for Wu, though, is understanding whether a similar neurological pathway exists in humans. If so, this research could prove ground-breaking in developing treatments for sleep disorders and related conditions, such as narcolepsy.
Initially, the researchers injected the mice with tracers into eleven regions of their brains that are known to induce sleep. These tracers travelled from the neurons receiving signals to their source. This then indicated the presence of 22 additional regions in their brains, which were connected to a minimum of four sleep-promoting areas.
For their study, the prime focus was the eleven previously-unidentified regions of the mice’s brain. First, the mice were split into eleven groups, with each group consisting of three-to-four mice. Then, using a technique called chemogenetics, the mice in each group were injected with a specialised drug that would activate a particular region of their brain. The region activated would be different in each group.
Key findings
After analysing the results, the researchers discovered an area that seemed to be the key behind the debt calculation – the thalamic nucleus. When neuron stimulation in this area was detected, the mice’s non-rapid eye movement (REM) sleep showed the greatest increase, going to almost double as compared to non-stimulated time.
The only caveat to the research, though, was that it took the mice longer to sleep after the neuron stimulation. During this time, they seemed to be preparing to rest.
“When you go to bed, you probably brush your teeth, you wash your face, you fluff your pillow or arrange your blanket and then go to sleep. Mice do something similar. They kind of groom their face, they clean their whiskers and then they fluff their nest up,” Wu was quoted as saying by New Scientist. This further explains that these neurons simply induce sleepiness, instead of triggering instant sleep.
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Further research
A secondary test backed up this idea. The researchers took a sample of six sleep-deprived mice, and deactivated the brain cells in their thalamic nucleus. This subsequently reduced the sleepiness of the mice, as they showed more activity while also spending less time nesting as compared to a control group. These six mice also got an average of 10% less non-REM sleep.
Studies also showed that these neurons automatically activated when a mouse was sleep-deprived, and their activity subsequently eased once it fell asleep. According to Wu, these findings point toward the thalamic nucleus as being the brain region inducing sleepiness, tracking sleep debt, and triggering restorative sleep to pay back the debt.
However, William Giardino, Assistant Professor of Psychiatry and Behavioral Sciences at Stamford, emphasised that the existence of a similar brain circuit in humans has not been established yet, nor has Wu’s research studied long-term effects.
(This article has been curated by Purv Ashar, who is an intern with The Indian Express)
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