Dreaming brain rhythms lock in memories
It is the clearest evidence to date that REM sleep is critical for memory.
By switching off certain brain cells, the researchers silenced a particular, rhythmic type of brain function - without waking the mice.
If they did this during REM sleep, the mice failed subsequent memory tests.
The research is reported in the journal Science .
REM sleep is the phase during which, at least in humans, dreams take place - but the question of whether it is important for settling new memories has been difficult to answer.
Recent studies have tended to focus on deep, non-REM sleep instead, during which brain cells fire in various patterns that reflect memory consolidation and "re-play" of the day's experiences.
During REM sleep, while our eyes flicker and our muscles relax, exactly what the brain is doing is something of a mystery. But it is a type of sleep seen across the animal kingdom, in mammals and birds and even lizards .
Especially in animals, REM phases can be quite fleeting. This and other complications have made it difficult to test what effect such sleep has.
Simply waking up humans or animals when they enter the REM phase, for example, causes stress and other problems that can confound any memory tests.
So Dr Sylvain Williams, from McGill University in Canada, working with colleagues at the University of Bern, Switzerland, decided to meddle directly with the sleeping brain.
"What we did was we used a technique, in mice, to solely disrupt REM sleep activity," Dr Williams told BBC News.
Using the system known as "optogenetics", he and his colleagues were able to control a particular population of brain cells in the mice, just by shining light through a tiny, implanted optical fibre.
Whenever they switched on the light, they drastically reduced a particular rhythm in the brain, called "theta oscillations".
And if that disruption was delivered during a mouse's REM sleep, there were consequences.
"Disrupting the activity only during REM sleep, and not other sleep, basically obliterates consolidation and memory formation," Dr Williams said.
For example, if it was shown one brand new object and one that it had seen the day before, the mouse would thoroughly investigate both, instead of concentrating - like a normal mouse would - on the unfamiliar one.
For one thing, the other, deeper phase of sleep is already known to be involved in memory consolidation. So what are their distinct jobs?
"I think at the moment we don't know the difference between the two phases," he said.
"It's an eye-opener to say that REM sleep has this very central role."
Whatever that role is, the new findings suggest it involves the oscillations that the scientists disrupted - in which brain cells synchronise their activity, leading to a widespread and measurable rhythm with, in this case, about seven beats per second.
That signature could be something to study in patients with dementia or other memory problems, Dr Williams said.
"It'd be interesting to see, for example, how this normal activity... might be affected, specifically, in Alzheimer's patients. To see if that contributes to memory impairments."
Dr Daniel Bendor, a neuroscientist at University College London who also studies memory formation in rodents, said the study was "a big step forward" in understanding REM sleep.
"After decades of work that's been inconclusive, they're showing that something important is happening," he said.
The Swiss-Canadian team had found a neat way of specifically disrupting REM sleep, Dr Bendor added.
"These are difficult experiments to do. It's a really impressive demonstration of a potential role of theta oscillations during REM sleep."
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I love to sleep. I come from a family where everyone, if the have the chance, takes a two or three hour afternoon nap. I know I do almost daily. I have always been fascinated with sleep. Why do we need to sleep? What happens to your brain when you do sleep? Why do we dream? Are they a needed part of brain health? Why do some people have vivid dreams that they remember (I do), while others can never remember their dreams (my wife never remembers her dreams? I know when I quit smoking and used the nicotine patch, one of the side effects listed was that it may cause vivid dreams and it did. Some medication do the same thing.
This research is investigating how waking people during a dream affects their memory and what implications sleep can have on Alzheimer's patients. It's exciting if they can find that different types of sleep in people can be an indicator of a predisposition to Alzheimer's and if it does, then they can diagnose and begin Alzheimer's treatment earlier, possibly delaying it's onset.
I also have come from a family of sleepers (even as a teen, I used to take an after school nap), but in my early 30's something went wonky with my sleep. I didn't know what, but it seemed like I wasn't dreaming. It also seemed like I was losing my mind.. I couldn't remember things and I was very crabby. I was constantly tired, but couldn't get in even a proper nap. I eventually went in for a sleep study.
It turned out that when I approached stage 4 sleep, I would start to have violent jerks that aroused me from sleep. It turned out that I had a sleep seizure disorder called essential nocturnal myoclonus. You can read about it here:
I am on medication now for over 20 years and I sleep and dream. It was a great relief to be able to sleep again. I can tell you first hand that my memory was shot without REM sleep.
I have always wonder what part dreams play in our brain health. I wonder WHY we have to sleep like every other creature. I wonder what our brains are doing while we sleep and why it seems that every creature needs to sleep also. I wonder if they dream also. I don't know of any land animal that doesn't need to sleep. We seem to have all evolved with it from the smallest insects to elephants.