August 3, 2022 – We tend to think that a good night’s sleep should be uninterrupted, but a startling new study from the University of Copenhagen suggests just the opposite: brief awakenings can be a sign that you’ve slept well.
The study, carried out on mice, revealed that the stress transmitter, norepinephrine, wakes up the brain several times a night. These “micro-awakenings” were linked to memory consolidation, which means they help you remember the events of the previous day. In fact, the more “awake” you are during a micro-arousal, the better the memory, research suggests.
“Every time I wake up in the middle of the night now, I think – ah, well, I probably just had a good, memory-boosting sleep,” says study author Celia Kjaerby, PhD, professor assistant at the university’s Center for Translational Neuromedicine. .
The findings add insight into what happens in the brain during sleep and may help point the way to new treatments for those who suffer from sleep disorders.
Waves of norepinephrine
Previous research has suggested that norepinephrine — a hormone that rises during stress but also helps you stay focused — is inactive during sleep. Thus, the researchers were surprised to see high levels in the brains of sleeping rodents.
“I still remember seeing the first traces showing brain activity of the norepinephrine stress system during sleep. We couldn’t believe our eyes,” says Kjaerby. “Everyone thought the system would be silent. And now we’ve discovered that it completely controls the microarchitecture of sleep.
These norepinephrine levels rise and fall like waves every 30 seconds during non-rapid eye movement (NREM) sleep. With each “peak”, the brain is briefly awake, and with each “valley”, it sleeps. Typically, these awakenings are so brief that the sleeping subject does not notice them. But the higher the rise, the longer the wake-up time – and the more likely the sleeper is to notice.
During valleys, or when norepinephrine drops, so-called sleep spindles occur.
“These are short oscillatory bursts of brain activity related to memory consolidation,” Kjaerby explains. Sometimes there is a “deep valley”, lasting 3-5 minutes, resulting in more sleep spindles. The mice with the deepest valleys also had the best memories, the researchers noted.
“We showed that the amount of these sleep spindle super-boosts, not REM sleep, defines how well you remember experiences you had before falling asleep,” Kjaerby says.
Deep valleys were followed by longer awakenings, the researchers observed. So the longer the valley, the longer the wake up – and the better the memory boost. This means that while restless sleep is not good, waking up briefly can be a natural part of memory-related sleep stages and can even mean that you slept well.
What happens in our brain when we sleep?
The findings match previous clinical data that shows we wake up around 100 times a night, mostly during stage 2 NREM sleep (the spindle-rich stage of sleep), Kjaerby says.
Still, more research into these small revivals is needed, Kjaerby says. She notes that Professor Maiken Nedergaard, MD, another author of this study, discovered that the brain cleans out waste through a system of flushing fluid.
“Why the fluid system is so active when we sleep remains a puzzle,” says Kjaerby. “We believe that these short awakenings could potentially hold the key to answering this question.”
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