We tend to think a good night's sleep should be uninterrupted, but surprising new research from the University of Copenhagen suggests just the opposite: Brief awakenings may be a sign you’ve slept well.
The study, done on mice, found that the stress transmitter noradrenaline wakes up the brain many times a night. These "microarousals" were linked to memory consolidation, meaning they help you remember the previous day's events. In fact, the more "awake" you are during a microarousal, the better the memory boost, the research suggests.
"Every time I wake up in the middle of the night now, I think — ah, nice, I probably just had great memory-boosting sleep," says study author Celia Kjaerby, PhD, an assistant professor at the university's Center for Translational Neuromedicine.
The findings add insight to what happens in the brain during sleep and may help pave the way for new treatments for those who have sleep disorders.
Waves of Noradrenaline
Previous research has suggested that noradrenaline — a hormone that increases during stress but also helps you stay focused — is inactive during sleep. So, the researchers were surprised to see high levels of it in the brains of the sleeping rodents.
"I still remember seeing the first traces showing the brain activity of the norepinephrine stress system during sleep. We could not believe our eyes," Kjaerby says. "Everyone had thought the system would be quiet. And now we have found out that it completely controls the microarchitecture of sleep."
Those noradrenaline levels rise and fall like waves every 30 seconds during non-rapid eye movement (NREM) sleep. At each "peak" the brain is briefly awake, and at each "valley" it is asleep. Typically, these awakenings are so brief that the sleeping subject does not notice. But the higher the rise, the longer the awakening — and the more likely the sleeper may notice.
During the valleys, or when norepinephrine drops, so-called sleep spindles occur.
"These are short oscillatory bursts of brain activity linked to memory consolidation," Kjaerby says. Occasionally there is a "deep valley," lasting 3 to 5 minutes, leading to more sleep spindles. The mice with the most deep valleys also had the best memories, the researchers noted.
"We have shown that the amount of these super-boosts of sleep spindles, and not REM sleep, defines how well you remember the experiences you had prior to going to sleep," says Kjaerby.
Deep valleys were followed by longer awakenings, the researchers observed. So, the longer the valley, the longer the awakening — and the better the memory boost. This means that, though restless sleep is not good, waking up briefly may be a natural part of memory-related sleep phases and may even mean you've slept well.
What Happens in Our Brains When We Sleep: Piecing It Together
The findings fit with previous clinical data that shows we wake up roughly 100-plus times a night, mostly during NREM sleep stage 2 (the spindle-rich sleep stage), Kjaerby says.
Still, more research on these small awakenings is needed, Kjaerby says. She notes that professor Maiken Nedergaard, MD, another author of this study, has found that the brain cleans up waste products through a rinsing fluid system.
"It remains a puzzle why the fluid system is so active when we sleep"” Kjaerby says. "We believe these short awakenings could potentially be the key to answering this question."
Sources
Celia Kjaerby, PhD, assistant professor, Center for Translational Neuromedicine, University of Copenhagen.
Nature Neuroscience: "Memory-enhancing properties of sleep depend on the oscillatory amplitude of norepinephrine."
