It might surprise you to know that even during the depths of sleep, your brain goes through brief bursts of wakefulness.
These “micro-arousals,” as they’re called, are too short to remember the next morning, but together they could help your brain consolidate your memories from the night before.
Among sleeping mice, researchers publish in the journal Natural neuroscience counted up to a hundred micro-awakenings occurring per night.
Far from interrupting a rodent’s rest, these occasional bursts of brain activity are part of what makes mammalian sleep so refreshing. So far, the results have only been shown in mice, but since they involve very basic biological mechanisms, the researchers say they could very well translate to humans as well.
In fact, the researchers believe that micro-arousals are “an intrinsic component of the normal micro-architecture of sleep” in mammals, allowing the memory system to “reset” multiple times per night. Once the brain goes back to sleep, it could lead to better overall memory consolidation.
“You may think that sleep is a constant state that you find yourself in and then wake up. But there’s a lot more to sleep than meets the eye,” says neuroscientist Celia Kjærby of the University from Copenhagen to Denmark.
“We learned that norepinephrine [also called norepinephrine] makes you wake up over 100 times a night. And that’s during perfectly normal sleep.”
Norepinephrine (NE) is a brain chemical and hormone associated with stress and wakefulness.
Previously, scientists thought that NE levels were largely stable during mammalian sleep, but the current mouse study indicates otherwise. In fact, the researchers found that the greater the swing of NE activity during mouse sleep, the better the overall rest.
In mouse models, NE activity was found to continuously increase and decrease during sleep, oscillating approximately every 30 seconds.
When NE levels dropped, the researchers noticed that short bursts of electrical activity called sleep spindles gradually built up in the brain. When NE levels began to rise, on the other hand, these spindles were terminated.
Sleep spindles are also closely linked to memory consolidation. In the past, fluctuations in spindle levels have been observed in sleeping mice and sleeping humans, but this is believed to be the first study to link cyclical cycles of NE activity to sleep spindles. and micro-awakenings.
In the experiments, sleeping mice with the largest NE activity dips were also better able to remember objects from the night before. Shorter descents also led to more micro-arousals rather than full arousals.
Most of these micro-arousals do not trigger conscious arousal, even briefly, but they do provide enough brain activity to reset sleep spindles.
“We found the essence of the part of sleep that makes us wake up rested and allows us to remember what we learned the night before,” says neuroscientist Maiken Nedergaard from the University of Copenhagen.
“We found that the refreshing part of sleep is driven by waves of noradrenaline. Very short awakenings are created by waves of noradrenaline, which are also so important for memory.”
Further research in mice and humans will be needed to explore how EN activity leads to better memory consolidation during sleep.
Additionally, the study authors argue that it is conceivable that oscillating NE levels work to reset sleep spindles in a way that “reduces the risk of awakenings and promotes micro-arousals.”
This way, memories can be stored more efficiently when sleep eventually resumes.
The study was published in Natural neuroscience.
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