Sleep: We all do it, a lot of us enjoy it, but no one knows exactly why we need it or what it does. We know that sleep deprivation can cause physical and psychological damage, yet it has also proved a very fast and effective treatment for depression (albeit usually only until the person falls asleep again). Basically, there's a lot about sleep that's still a mystery.
Christoph Nissen, a psychiatrist and psychotherapist at the University Medical Center Freiburg in Germany, wants to understand more about the function of sleep in order to unravel potential mechanisms of related disorders and treatments. To do so, he and a team of researchers recently measured people's brain activity after a night of sleep and a night of no sleep. They found several key differences in the sleep-deprived participants. Their results are published in Nature Communications.
Nissen pointed out that we spend around a third of our lives asleep, and that sleep is also prevalent across the animal kingdom. "Still we do not really know why we spend such a long time of our lives in this inactive state, so sleep must have a very important function," he said. "Otherwise it's just a very big mistake that evolution made."
"Neurons that fire together, wire together"
In the new study, the researchers looked specifically at synaptic plasticity—the change in the strength of connectivity between neurons, which sleep has been shown to modulate. They looked at both homeostatic plasticity, which is the overall strength of connections in the brain, and associative plasticity, which refers to the selective strengthening of connections in response to new information, such as learning a specific skill that requires relevant neurons to activate together in order to encode memory. As Nissen put it, "Neurons that fire together, wire together."
The researchers tested this in 20 participants using EEG and transcranial magnetic stimulation (TMS), which they used to apply a magnetic pulse to the motor cortex in order to induce a hand movement. They found that, after a night of sleep deprivation, participants required a significantly lower pulse to elicit the same hand movement. This suggests that the overall excitability of the brain is higher with sleep deprivation, having built up through the day.
That might initially sound like a good thing, but a generally more excitable brain isn't good at that all-important, memory-building associative plasticity. "The thing is that at the same time, in these participants where the brain was more excitable after sleep deprivation, we observed that the inducibility of associative plasticity was occluded," said Nissen. "So this is the downside."
"Sleep kind of wipes out or cleans up the connections in the brain to enable the novel acquisition of information"
They tested this inducibility by electrically stimulating a nerve in participants' arms (which then sends information to the brain) just before administering a TMS pulse—essentially mimicking neurons firing together. After doing this many times, you'd expect the relevant synapses to strengthen, something called long-term potentiation. This kind of plasticity was decreased after sleep deprivation, compared to after sleep. As it's a basic mechanism of learning and memory, that's not so great. Participants also performed less well in a word pairing test after sleep deprivation.
Taking these two main observations, the researchers propose an interplay between the two types of plasticity, with an optimal window for strengthening new connections occurring as the overall strength of connections builds. They propose that sleep effectively "resets" the overall strength, allowing this window to occur.
"Basically sleep kind of wipes out or cleans up the connections in the brain to enable the novel acquisition of information," Nissen said. "This might be a very fundamental function of sleep." Essentially, you need that recalibration of sleep for your brain to get back to the sweet spot of learning new things and forming memories.
Moving forward, he hopes this research may provide insight into developing countermeasures for sleep deprivation and treatments for conditions such as depression.
And more fundamentally, it could shed more light on why we sleep in the first place: "It speaks against the idea that sleep is just a passive waste of time."