Scientists probing the mysterious machinations of the brain during sleep have made a fascinating discovery around how its chemistry changes throughout the night. The research centers on the levels of a stress hormone called noradrenaline, which fluctuates wildly and awakens our brain without us even realizing it and may have implications for the way we retain memories.
“You may think that sleep is a constant state that you are in, and then you wake up. But there is a lot more to sleep than meets the eye," explains Assistant Professor Celia Kjærby from the University of Copenhagen. "We have learned that noradrenaline causes you to wake up more than 100 times a night. And that is during perfectly normal sleep."
Kjærby and her colleagues made this discovery through experiments on mice. Optical fibers attached to LEDs and genetically-engineered light receptors were inserted into the brains of the rodents, which enabled levels of noradrenaline to be tracked as they slept. This neurotransmitter is associated with stress, adrenaline and our ability to focus. Previous research has implicated the noradrenaline system in the relationship between breathing-based meditation and cognitive function and raised the prospect of leveraging it in next-generation treatments for ADHD
Prior studies have also suggested that noradrenaline is inactive during sleep. But by monitoring its levels in sleeping mice, the scientists have found that to be far from the case. And because the study focused on the same biological mechanisms shared by all mammals, the scientists say the observations translate to humans "in all probability."
Noradrenaline concentrations were seen increasing and decreasing in waves during sleep. The scientists observed a clear relationship between noradrenaline levels and the degree of wakefulness, which is constantly shifting throughout the night. Low levels corresponded with a sleeping state, and when levels peaked, the brain would briefly awaken.
“Neurologically, you do wake up, because your brain activity during these very brief moments is the same as when you are awake," said second author of the study, Mie Andersen. "But the moment is so brief that the sleeper will not notice.”
In further experiments, the scientists used implants to actually manipulate the amplitude of the noradrenaline waves. In doing so, they were able to show that mice with the highest number of deep noradrenaline valleys performed best in memory tests. These involved having the mice sniff objects, go to sleep, and then return to them to see what they remembered.
“The mice developed 'super memory,' said Kjærby. "They had less trouble remembering things they had learned the previous day. Of course, this suggests that the noradrenaline dynamic strengthens the sleep processes which affect our memory."
Some antidepressants will actually increase levels of noradrenaline, which the scientists point out could impact on our memory in light of these new findings. This suggests the development of alternative drugs could be a healthier path forward in treating depression. More broadly, the study sheds important new light on the benefits of sleep when it comes to memory.
“We have found the essence for the part of sleep that makes us wake up rested and which enables us to remember what we learned the day before," said Maiken Nedergaard, who led the study. 'We have found that the refreshing part of sleep is driven by waves of noradrenaline. The very short awakenings are created by waves of norepinephrine, which are also so important for memory. You could say that the short awakenings reset the brain so that it is ready to store memory when you dive back into sleep."
The research was published in the journal Nature Neuroscience.
Source: University of Copenhagen