Sleep Resets Brain Connections Crucial For Memory and Learning

Sleep Resets Brain Connections Crucial For Memory and Learning.

The reason we spend a third of our lives asleep has so far resisted scientific explanation. Research into the impact of sleepless nights on brain function has shed fresh light on the mystery – and also offered intriguing clues to potential treatments for depression. In a study published on Tuesday, researchers show for the first time that sleep resets the steady build-up of connectivity in the human brain which takes place in our waking hours. The process appears to be crucial for our brains to remember and learn so we can adapt to the world around us. The loss of a single night’s sleep was enough to block the brain’s natural reset mechanism, the scientists found. Deprived of rest, the brain’s neurons seemingly became over-connected and so muddled with electrical activity that new memories could not be properly laid down.

But Christoph Nissen, a psychiatrist who led the study at the University of Freiburg, is also excited about the potential for helping people with mental health disorders. One radical treatment for major depression is therapeutic sleep deprivation, which Nissen believes works through changing the patient’s brain connectivity. The new research offers a deeper understanding of the phenomenon which could be adapted to produce more practical treatments. The results are a boost for what is called the synaptic homeostasis hypothesis of sleep, which was developed by scientists at the University of Wisconsin-Madison in 2003. It explains why our brains need to rest after a day spent absorbing all manner of information, from the morning news and the state of the weather, to a chat over lunch and what we must buy for tea. Known more simply as SHY, the hypothesis states that when we are awake, the synapses that form connections between our brain cells strengthen more and more as we learn and eventually saturate our brains with information. The process requires a lot of energy, but sleep allows the brain to wind down its activity, consolidate our memories, and be ready to start again the next morning.

Writing in the journal Nature Communications, Nissen describes a series of tests that 11 men and nine women aged 19 to 25 took part in, either after a good night’s sleep, or after a night without sleep. In contrast, the sleep-deprived brain becomes noisy with electrical activity and so feeble at laying down memories that the process is all but blocked. The consequences of sleep loss were clear in a simple memory test, with tired volunteers faring worse than those who were well-rested. Blood samples taken from volunteers in the study showed that sleep deprivation lowered levels of a molecule called BDNF, or brain-derived neurotrophic factor, which regulates synaptic connections in the brain.

But Nissen is more excited about the study’s implications for understanding therapeutic sleep deprivation and its impact on depression. “If you deprive people with major depression of sleep for one night, about 60% show a substantial improvement in mood, motivation and cognitive function. We think it works by shifting these patients into a more favourable state,” he said. Though striking when it works, therapeutic sleep deprivation is not much use because many patients relapse after the subsequent night’s sleep. But that is not the point, Nissen says. “It proves that it’s possible to shift a person’s mood from one state to another within hours. The idea is that we use sleep and sleep deprivation to understand the brain and develop new treatments. If you think about antidepressants or psychotherapy, it can take weeks or months to see any effects.”

Credit: Ian Sample for The Guardian Neuroscience (Abridged) 23 August 2016