It might surprise you to know that even during the depths of sleep, your brain cycles through brief bouts of wakefulness.
These 'micro-arousals', as they are known, are too short to be remembered the next morning, but together they could help your brain consolidate your memories from the day before.
Among sleeping mice, researchers publishing in the journal Nature Neuroscience have counted as many as a hundred micro-arousals occurring a night.
Far from interrupting a rodent's rest, these occasional spurts of brain activity are part of what make mammalian sleep so refreshing. So far, the results have only been shown in mice, but since they involve some very basic biological mechanisms, the researchers say they could very well translate to humans, too.
In fact, researchers think micro-arousals are "an intrinsic component of normal sleep micro-architecture" in mammals, allowing the memory system to 'reset' multiple times a night. Once the brain falls back asleep, this could lead to better memory consolidation overall.
"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 neuroscientist Celia Kjærby from the University of Copenhagen in Denmark.
"We have learned that noradrenaline [also called norepinephrine] causes you to wake up more than 100 times a night. And that is during perfectly normal sleep."
Norepinephrine (NE) is a brain chemical and hormone associated with stress and wakefulness.
Previously scientists thought NE levels were largely stable during mammalian sleep, but the current study on mice indicates otherwise. In fact, researchers found the greater the swing in NE activity during mouse sleep, the better the rest overall.
In mouse models, NE activity was found to rise and fall continuously during sleep, oscillating every 30 seconds or so.
When NE levels descended, 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 connected to memory consolidation. In the past, fluctuations in spindle levels have been found in sleeping mice and in sleeping humans, but this is thought to be the first study to link cyclic cycles in NE activity to sleep spindles and micro-arousals.
In experiments, the sleeping mice with the greatest troughs in NE activity were also better able to remember objects from the day before. Shorter descents also led to more micro-arousals rather than full awakenings.
Most of these micro-arousals don't trigger conscious wakefulness, even briefly, but they do provide enough brain activity to reset sleep spindles.
"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," says neuroscientist Maiken Nedergaard from the University of Copenhagen.
"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."
Further research on mice and in humans will be needed to explore how NE activity drives improved memory consolidation during sleep.
Nevertheless, the authors of the study 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 Nature Neuroscience.