That being said, there’s an interesting new study showing that people can learn new information when they sleep. Earlier work tells us that we can certainly strengthen existing memories when we slumber, but actually adding new information is different. And retaining that information when we wake up, even if we have no actual awareness of what we learned, is just plain cool.
I wrote about the new study for The Scientist. Head on over.
Image by Alessandro Zangrilli
In a lab in Atlanta, a group of flies is sleeping fitfully. Their naps are fragmented, and their legs are twitching. Their behaviour is uncannily similar to people who have a condition called restless leg syndrome (RLS). When such people are awake, they experience uncomfortable sensations in their limbs that compel them to move to get some relief. Their sleep, which is fragmented and disturbed, is characterised by the same involuntary movements.
There’s a good reason for these similarities. Amanda Freeman from the Emory University School of Medicine has engineered the flies so that they have a faulty copy of BTBD9, a gene that has been linked to RLS in humans. The fact that they show the same constellation of symptoms strongly suggests that this gene is genuinely involved in the condition.
How does an ostrich sleep? Almost imperceptibly, it seems. Even though an ostrich might be sound asleep, it can look wide awake or, at most, a little drowsy. John Lesku from the Max Planck Institute of Ornithology discovered this by fitting six ostrichers with “Neurologgers”, electrode-laden helmets that measures their temperature, brain activity, eye movements and neck muscle contractions.
The video above shows three of the birds cycling through two different types of sleep. The first is called ‘slow wave sleep’ or SWS, where the ostriches’ brain waves are slow and strong. Even though this is typically known as deep sleep, the birds look alert. They stay still, but their eyes are open and their necks upright. Nonetheless, the readings from the Neurologgers clearly showed that they were asleep.
In the second phase, known as ‘rapid eye movement’ or REM sleep, the ostriches’ brain waves are fast and weaker. Now, the birds shut their eyes, which move rapidly behind closed eyelids. They necks also start to droop and sway, righting themselves with awkward jerks like people falling asleep at a talk. Biologists have previously interpreted this as a sign of a tired ostrich. That’s partly right, although the animal is already asleep rather than on its way.
Many mental disorders can disrupt the sweet embrace of a long, continuous sleep, including alcoholism, depression, Alzheimer’s and parenthood. And that’s bad news. We know that a good night’s sleep helps to solidify our memories of the previous day’s experiences. And according to a new study, we need a certain amount of continuous sleep for those benefits to kick in.
From an evolutionary point of view, it seems strange that we sleep for hours on end. Rather than leaving ourselves unresponsive and vulnerable for large chunks of time, why not simply sleep over several shorter fragments?
This is not an easy question to answer. Until recently, it has been all but impossible to break up the continuity of sleep without also affecting its quality, or stressing out the animals in question. But Luis de Lecea from Stanford University has found a way. He has engineered mice with in-built silent alarm clocks. These animals can be woken up at will with a pulse of light delivered directly to their brains.
The good news for all new parents is that scientists have found a way of sending individuals straight to sleep by turning up the thermostat. The bad news is that it only works in flies. Alas, this technique is not going to solve anyone’s sleepless nights, but it could tell us something about why we sleep at all.
Every animal, or at least every one with a brain, needs to sleep, but it’s still not entirely clear why. William Dement, who has been studying sleep for six decades, once said, “As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.” As Daniel Bushey from the University of Wisconsin writes, “Sleep is perhaps the only major behaviour still in search of a function.”
We all know people who look like they can nod off with their eyes open. These exceptions aside, we generally think of sleep as a switch with two settings – you’re either asleep or awake. But Vladyslav Vyazovskiy from the University of Wisconsin-Madison has found that sleep is more complicated than that.
By studying the brains of sleep-deprived rats, Vyazovskiy found that individual neurons can effectively fall asleep, going “offline” while those around them carrying on firing. Even if the rats are awake, parts of their brain can be taking a nap. What we know as “sleep” is the global version of something that happens throughout the brain at a local level.
Caves are dark, sheltered and often quiet. They’re seemingly ideal places for a bit of a nap. But for a small Mexican fish, they have done exactly the opposite. As a result of life in dark caves, the blind cavefish has evolved sleeplessness, on at least three separate occasions. They don’t go entirely without sleep, but they doze far less than their surface-dwelling relatives.
The blind cavefish (Astyanax mexicanus) is a sightless version of a popular aquarium species, the Mexican tetra. They live in 29 deep caves scattered throughout Mexico, which their sighted ancestors colonised in the middle of the Pleistocene era. In this environment of perpetual darkness, the eyes of these forerunners were of little use and as generations passed, they disappeared entirely. Today, the fish are born with eyes that degenerate as they get older. Eventually, their useless husks are covered by skin.
They went through other changes too. For example, their skin lost its pigment so they are all pinkish-white in colour. And now, Erik Duboué from New York University had found that they also sleep less than their relatives on the surface.
The business of encoding new memories is more like writing a document on a computer than inscribing words onto paper. Until you save the file, there’s a chance that you could lose the information. This vulnerable window can last for a couple of days. Only after that point does the memory become strong and long-lasting. This is called ‘consolidation’.
It’s not a permanent state. Whenever we remember something, the fragile window reopens. Again, it’s more like opening a computer document than getting notes out of a drawer. You could easily add, edit or delete information at a flick of a key. Every time we bring back an old memory, we run the risk of changing it. Again, it takes a while for this window of opportunity to close, for the reactivated memory to strengthen once more. This is called ‘reconsolidation’.
In the last week, two groups of scientists have found two very different ways of boosting both processes, to produce stronger memories.
It seems obvious that thinking about something will help you to remember it better, but it might be more surprising to know that this process works even more efficiently when we’re asleep. Erin Wamsley from Harvard Medical School has shown that people who are trained to navigate a virtual maze learn the best route through it more quickly if they dream about their experiences.
The last decade of research has clearly shown that sleep is one of the best aide memoires that we have. During this nightly time-out, our brain can rehearse information that it has picked up during the day and consolidate them into lasting memories. Wamsley’s new study supports that idea but it also shows that dreaming while you nap can strengthen our memories even further.
She asked 99 volunteers to learn the layout of a complex virtual maze so that they could reach a specific landmark after being dropped at a random starting point. Five hours later, they were tested again. Those who had stayed awake in the intervening time beat their previous times by 26 seconds, but those who had had a 90-minute nap improved by a whopping 188 seconds.
But those who dreamt about the task fared even better. Wamsley either asked her recruits directly about whether they dreamt about the labyrinth, or asked them to give an open-ended report of everything that was going through their mind while they were asleep. Either way, those who had thought about the maze during their short nap improved far more than those who didn’t. They also beat those who mentally replayed their training again while awake. These striking results suggest that there’s something special about the mental rehearsals that happen during dreaming sleep.
In my final year of university, with exam deadlines looming and time increasingly fleeting, I considered recording some of my notes and playing them over while I was asleep. The concept of effectively gaining 6 extra hours of revision was appealing, but the idea didn’t stick – it took too long to record the information and the noise stopped me from sleeping in the first place. And the whole thing had a vague hint of daftness about it. But a new experiment suggests that the idea actually has some merit, showing that you can indeed strengthen individual memories by reactivating them as you snooze.
Sleep is a boon to newborn memories. Several experiments have shown that sleep can act as a mental cement that consolidates fragile memories into stable ones. But John Rudoy from Northwestern University wanted to see if this process could be taken even further by replaying newly learned information while people slept.
He asked a dozen volunteers to remember the positions of 50 different objects as they appeared on a screen. The items, from kittens to kettles, were all accompanied by a relevant noise, like a meow or a whistle. Shortly after, the recruits all had a short nap. As they slept, Rudoy played them the sounds for 25 of the objects, against a background of white noise. When the volunteers woke up, they had to place each of the 50 objects in the right position, and they were marked on how close they came to the actual target.
The results were very clear – the volunteers positioned the objects around 15% more accurately if they’d heard the relevant sounds while they slept. Although the sleep sounds didn’t work for everyone, the majority of the participants – 10 out of 12 – benefited in some way. And none of them knew they heard anything at all while they slept. When they were told this and asked to guess which sounds they heard, they didn’t do any better than chance.
To show that this isn’t just a general benefit of revision, whether conscious or not, Rudoy did a similar experiment. This time, his volunteers heard the noises after they had first seen the objects but while they were still awake. This group proved to be no better at remembering the items’ locations than those who didn’t hear the second round of sounds.
Finally, to understand what was going on in the brains of the slumbering recruits, Rudoy used electroencephalograms (EEG) to measure the electrical activity in the heads of 12 fresh volunteers. He showed that people who were better at remembering the objects’ positions after their nap were also those who showed the most brain activity when they heard the sounds Rudoy thinks that hearing the sounds during sleep prompted the brain to rehearse and strengthen associations between the objects and their locations.
Some people think that sleep improves memories in a general way, by making our brains more flexible and easing the incorporation of new information. But these simple experiments show that the benefits can be very specific indeed. It’s not only possible to strengthen specific and individual memories by providing the right triggers, but we get the opportunity to do so every single night.
More on sleep: