This Is a Good Thing
By UNIVERSITY OF COPENHAGEN - THE
FACULTY OF HEALTH AND MEDICAL SCIENCES
“Oh
no, it is not time to wake up yet,” you think, fearing that you will need lots
of coffee to stay awake the following day.
The
majority of individuals feel that a good night’s sleep must be uninterrupted.
That is why waking up in the middle of the night when all you want to do is
sleep can be incredibly irritating.
According
to a new study from the University of Copenhagen, the stress transmitter
noradrenaline leads you to wake up often throughout the night. But don’t be
concerned. It’s all part of a regular, good night’s sleep, and it may even
indicate that you slept well.
“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. We have learned that noradrenaline causes you to wake up more than 100 times a night. And that is during perfectly normal sleep,” says Assistant Professor Celia Kjærby from the Center for Translational Neuromedicine, who is one of the first authors of the study.
Despite
the fact that noradrenaline technically causes the brain to wake up more than
100 times every night, we do not consider this to be waking up.
“Neurologically,
you do wake up, because your brain activity during these very brief moments is
the same as when you are awake. But the moment is so brief that the sleeper
will not notice,” explains Ph.D. Student Mie Andersen, the second first author
of the study.
Despite
the fact that the researchers analyzed mice, their results are most likely
applicable to humans since they focused on fundamental biological mechanisms
shared by all mammals.
What is Noradrenaline?
Noradrenaline
is a stress hormone and transmitter substance, which i.a. is associated with
the body’s fight or flight response. It is related to adrenaline, and levels
may increase during stress, but it also helps you stay focused.
The stress transmitter noradrenaline affects the sleep waves
Professor
Maiken Nedergaard, who has led the study, sees the new finding as an important
piece of the puzzle to understanding what happens in the brain when we sleep.
“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. 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,” says Maiken Nedergaard and adds:
“You
could say that the short awakenings reset the brain so that it is ready to
store memory when you dive back into sleep.”
We
will return to the subject of memory shortly.
Previous
research has suggested that noradrenaline, which is associated with stress, is
inactive during sleep. Therefore, the researchers were surprised to see how
active noradrenaline really is during sleep.
The
new study shows that when we sleep the level of noradrenaline in the body is
constantly increasing and decreasing in a wavelike pattern. High levels of
noradrenaline mean that the brain is briefly awake, while low levels of
noradrenaline mean that you are asleep. That is, your noradrenaline levels and
degree of ‘awakeness’ are connected and constantly changing.
“Approximately
30 seconds pass from one ‘top’ to the next, which means that your noradrenaline
levels are constantly changing. At the same time, we could tell that the deeper
the ‘valley’, i.e. the better the sleep, the higher the subsequent top, and the
higher degree of awakening,” says Mie Andersen.
“This
shows that perhaps you do not need to worry if you wake up at night. Of course,
it is not good to be sleepless for extended periods, but our study suggests
that short-term awakenings are a natural part of sleep phases related to
memory. It may even mean that you have slept really well,” Celia Kjærby adds.
What the researchers did
Microscopic
optical fibers made of glass and genetically manipulated ‘light receptors’ were
inserted into the brains of the test mice. The optical fibers were attached to
cables, including an LED light source.
Subsequently,
the researchers measured the here-and-now levels of noradrenaline while the
animals slept and compared it to the electrical activity in their brains. This
was where they spotted the high levels of noradrenaline.
The
researchers then conducted memory tests by using the implanted equipment to
increase the amplitude of the noradrenaline waves, improving the animals’
memory.
The mice developed “super memory”
It
is a well-known fact that sleep is good for us – in a number of ways. It
removes waste products, prevents Alzheimer’s, and
improves our memory.
The
latter was also a focus of this study, and the findings suggest that the mice
with the highest number of deep noradrenaline valleys were also the ones with
the best memory.
“The
mice developed ’super memory’. They had less trouble remembering things they
had learned the previous day. Of course, this suggests that the noradrenaline
dynamic strengthens the sleep processes which affect our memory,” says Celia
Kjærby.
First,
the mice were allowed to sniff at two identical objects. They were then put to
sleep, and once awake they were returned to the objects. However, one of the
two objects had now been replaced by a new one. The mice who had seen the
highest number of noradrenaline valleys were more inclined to study the new
object, which suggests that they remembered having seen a different object last
time.
New perspectives on the use of noradrenaline in antidepressants
Besides
increasing our knowledge of the engine room of sleep, the new study provides
food for thought when it comes to antidepressants.
“Some
forms of antidepressants increase the level of noradrenaline in the body, which
increases the risk that you will see fewer deep sleep valleys. Our study shows
that this is likely to affect your memory,” Celia Kjærby says and adds:
“That
is why we need to focus attention on how different types of medication
regulating the level of noradrenaline in the body affect our sleep. In the
future, we should seek to develop drugs that do not affect the noradrenaline
waves during sleep.”
Reference:
“Memory-enhancing properties of sleep depend on the oscillatory amplitude of
norepinephrine” by Celia Kjaerby, Mie Andersen, Natalie Hauglund, Verena
Untiet, Camilla Dall, Björn Sigurdsson, Fengfei Ding, Jiesi Feng, Yulong Li,
Pia Weikop, Hajime Hirase and Maiken Nedergaard, 7 July 2022, Nature Neuroscience.
DOI: 10.1038/s41593-022-01102-9