(ORDO NEWS) — It may surprise you that even during deep sleep, your brain cycles through short bursts of wakefulness.
These “microarousals,” as they’re called, are too short to remember the next morning, but together they can help your brain solidify memories from the day before.
Among sleeping mice, the researchers, who published data in the journal Nature Neuroscience, counted up to a hundred microarousals occurring per night.
These episodic bursts of brain activity do not interrupt the rodent’s rest, but are part of what makes mammalian sleep so refreshing. So far, the results of the study have only been obtained in mice, but because they affect some basic biological mechanisms, the researchers say they could very well be applied to humans.
In fact, the researchers believe that microarousals are “an essential component of the normal sleep microarchitecture” in mammals, allowing the memory system to “reboot” several times a night. When the brain goes back to sleep, it can lead to better memory retention in general.
“You may think that sleep is a constant state that you are in and then wake up. But sleep is much more than meets the eye,” explains neuroscientist Celia Kjerby from the University of Copenhagen in Denmark.
“We learned that norepinephrine [also called norepinephrine] makes you wake up more than 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 sleep in mammals, but the current study in mice suggests otherwise. In fact, the researchers found that the more fluctuations in NE activity during sleep in mice, the better overall rest.
In mouse models, NE activity has been found to rise and fall continuously during sleep, fluctuating every 30 seconds or so.
When NE levels dropped, the researchers noticed that short bursts of electrical activity, called sleep spindles, gradually built up in the brain. When the level of NE began to rise, these spindles stopped.
Sleep spindles are also closely related to memory consolidation. In the past, fluctuations in spindle levels have been found in sleeping mice and in sleeping humans, but this study is believed to be the first to link cycling of NE activity to sleep spindles and micro-arousals.
In the course of the experiments, sleeping mice with the greatest declines in NE activity also remembered better the objects found the day before. Shorter descents also resulted in more micro-awakenings rather than full revivals.
Most of these micro-awakenings do not induce conscious wakefulness, even for a brief moment, but they provide enough brain activity to reset the spinal spindles.
“We have found the essence of the part of sleep that makes us wake up refreshed and allows us to remember what we learned the day before,” says neuroscientist Maiken Nedergaard from the University of Copenhagen.
“We found that the refreshing part of sleep is driven by waves of norepinephrine. Very short awakenings are driven by waves of norepinephrine, which is also very important for memory.”
Further studies in mice and humans will be required to elucidate how NE activity improves memory consolidation during sleep.
However, the authors of the study state that it can be hypothesized that fluctuations in NE levels help reset sleep spindles, which “reduces the risk of awakenings and promotes micro-arousals.”
In this way, memories can be stored more efficiently when sleep eventually resumes.
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