It’s time for bed, and what better way to ensure a comfortable sleep than snuggling into a cocoon of your own mucus?
Humans could probably think of some alternatives, but for many coral reef fish, mucus sleeping bags are all part of a good night’s rest. Many species of parrotfish and wrasse belch out their own cocoons every night, covering themselves in under an hour. And Alexandra Grutter from the University of Queensland thinks she knows why – the mucus deters vampires.
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:
When we think of memory aids, we consider repeating what we’ve learned, using clever mnemonics, or breaking information down into bite-size chunks. But one of the best memory aids we have available to us is something we all do on a daily basis – sleep. Studies have found that sleep enhances our memories of facts and physical skills alike. It can even help us remember movements that we see others do.
But this only works within a short window. Ysbrand van der Werf from the Netherlands Institute of Neuroscience found that people who saw a video of someone tapping keys on a laptop remembered the sequence more accurately if they slept on it within 12 hours. Any longer than that, and the snoozing didn’t boost their recall.
Van der Werf showed the video to 128 volunteers and then tested them on either the same finger-tapping sequence or a different one. The gap between video and test was either 12 or 24 hours, and some of the volunteers were allowed to sleep during the interval while others were not.
If the test sequence didn’t match the ones they saw, all the recruits did equally well. But if the sequence was the same, those who managed to sleep within the first 12 hours stood out – they were 22% faster and made 42% fewer errors than their peers who either didn’t sleep or who slept later. They even improved whether they had their naps during the day or in the evening.
These results parallel those from experiments where people actually had a chance to practice new skills before their naps. The big difference here is that the improvements came only after watching movements rather than actually performing them.
Van der Werf confirmed that by taking great care to ensure that his volunteers weren’t actually trying out the keystrokes for themselves. While watching the video, they had to tap two different keys to keep their fingers busy. Van der Werf even measured the muscle activity in the arms of seven volunteers to rule out the possibility that they were making subtle, unnoticed finger movements.
If it’s not to do with practice, it’s not to do with memorising the digits themselves or the position of the keys either. If the volunteers just saw the numbers flash up on screen, or if they saw coloured squares light up in the same position as the relevant keys, they didn’t become more accurate or faster when they had to replicate the sequence. They needed to actually see someone else doing it.
Van der Werf thinks that the recruits probably imagined their finger movements while watching the video, even if they didn’t actually try them out. It may even involve the mirror neurons that fire when an individual performs an action and when it sees someone else doing the same action (although mirror neurons have only been properly found in monkeys, and not humans).
Either way, the results highlight the importance of a good sleep when people are trying to pick up new physical skills. This could be especially important for people who can’t possibly to practice the movements in question, such as those who have suffered a stroke or broken a limb. And clearly the most important implication is that the next time I see someone doing parkour, I will immediately lie down and have a little nap. When I wake up, I will be Batman. SCIENCE!
Reference: PNAS doi:10.1073_pnas.0901320106
More on memory:
The German chemist Friedrich Kekule claimed to have intuited the chemical structure of the benzene ring after falling asleep in his chair and dreaming of an ouroboros (a serpent biting its own tail). He’s certainly not the only person to have discovered a flash insight after waking from a good sleep. In science alone, many breakthroughs were apparently borne of a decent snooze, including Mendeleyev’s creation of the Periodic Table and Loewi’s experiments on the transmission of nervous signals through chemical messengers.
Most of us have tried sleeping on a difficult problem before and using an elegant experiment, Denise Cai from the University of California in San Diego has shown that this old technique really does have merit to it. She found that our brains are better at integrating disparate pieces of information after a short bout of REM (rapid eye movement) sleep – a deep, dream-rich slumber that involves a rapid fluttering of the eyes. Cai thinks that REM sleep catalyses the creative process by allowing the brain to form connections between unrelated ideas.
Cai is by no means the first person to link sleep or dreaming to creative revelations, but she is one of the few to test it directly through experiments. She asked 77 people to complete a task, where they were given a list of three words and had to find a fourth that was linked to all three. For example, ‘cookie’, ‘heart’ and ‘sixteen’ are all associated with ‘sweet’. In each example of this ‘Remote Associates Test‘ (RAT), the missing fourth word has a different relationship to each of the three targets.
Most of us start to tire after about half a day without any sleep. Staying awake for five in a row would be extremely difficult and even if you could manage it, you’d be a physical and mental wreck by the end. But not all animals suffer from the same problem. A dolphin can stay awake and alert for at least 5 days straight, chaining together all-nighters without any noticeable health problems or loss of mental agility.
The two halves of a dolphin’s brain can sleep in shifts, “shutting down” one at a time so that the animal is always half-awake. They can truly sleep with one eye open, an essential skill for an animal that has to be constantly watching for predators and timing its breaths.
Previously, Sam Ridgway from the US Navy Marine Mammal Program found that bottlenose dolphins could respond to a distinct noise for 5 straight days without any dip in accuracy. He trained a female called Say to recognise occasional 1.5-second beeps amidst a background of shorter 0.5-second ones. When she heard the longer tone, she pressed a lever for food. Say was slower to respond at night, but overall, her reaction times didn’t slow over the five day run.
It was an impressive performance, but one that didn’t require much in the way of thought. This time, Ridgway wanted to see if the dolphin’s mental skills would take a hit after five days of continual vigilance. To do that, he trained Say, and another male called Nay, to make different noises for two different visuals – a whistle for a single, vertical, green bar of light, or a burst of sonar pulses for three, horizontal, red bars. Incidentally, dolphins are colour-blind; the colours were for the scientists’ benefit.