Category: Sleep and dreaming

Inverse zombies studied using anaesthesia

Hospital medicine takes a pretty crude approach to consciousness. You’re considered mentally AWOL if you don’t respond to simple commands or physical prodding. But studies of post-operative patients have found that many of them recall having dreamt during anaesthesia. And in some disturbing cases they’ve even felt pain or heard the surgeons talking. This suggests that it’s possible to be outwardly dead to the world, but conscious inside (locked-in patients and imaging studies of brain-injured patients in a persistent vegetative state also imply the same thing). Researchers have nicknamed people in this state “inverse zombies” – a play on the standard philosophical zombie concept, in which a person may appear to be outwardly conscious, but is in fact, dead inside.

A problem with much of the research into “inverse zombies” is that it’s been conducted opportunistically in hospitals. The experimental set-up is messy, the patients have a variety of health complications, and they’ve often been given a cocktail of anaesthetic drugs. These studies have found rates of awareness during anaesthesia at around 0.023 to 1 per cent and rates of anaesthesia dreaming at rates of 6 to 53 per cent.

Now Valdas Noreika and his collaborators have performed a carefully controlled lab study of subjective (or “phenomenal”) consciousness during anaesthesia, with the help of 40 healthy male university students. These brave souls were given progressively higher doses of one of four different anaesthetic drugs: dexmedetomidine; propofol (the drug that tragically killed Michael Jackson, who was using it as a sleeping aid); sevoflurane; and xenon. Dexmedetomidine and propofol are given intravenously; the other two are inhaled.

After the doping had begun, the researchers gave the participants the verbal command “Open your eyes!” at five minute intervals. Once a participant stopped responding they were considered to be unconscious in the traditional medical sense and the dose was gradually lowered until they responded again. Throughout, the researchers recorded the surface electrical activity from the front of the participants’ brains using a “Bispectral Index Monitor (BIS)” – a form of electroencephalography (EEG), which provided an objective measure of the depth of sedation.

The induction phase – from the last response to “Open your eyes!” to the loss of responsiveness – lasted typically from around 5 to 10 minutes; the period of sedation or loss of responsiveness itself lasted around 10 minutes; this was followed by a 2 minute recovery phase and then 5 minutes of EEG scanning. At this point, the participants were interviewed about their subjective experiences during the time they were knocked out.

The key finding is that dreams or sensations were experienced during nearly 60 per cent of the anaesthesia sessions. These ranged from perceptual sensations (including “quick visual experiences”; out-of-body sensations; an altered sense of time); dream-like experiences (had a fragmentary dream about “a trip in Eastern Europe” said one participant); vision-based dreams related to the lab situation (“one of the nurses got suspended from her work”); and dreams with auditory content based on the lab situation (“a friend’s roommate … sitting next to me here in the lab, telling me we have to go to the city”). Sometimes these experiences were accompanied by negative emotions (“a bit anxious”); other times positive (“felt extraordinarily good”). The type of experiences didn’t vary with the particular anaesthetic given.

Noreika and his team say these findings are important because they highlight the inadequacy of the standard medical definition of loss of consciousness (i.e. a loss of responsiveness), which is used in many anaesthesia-based studies into the neural correlates of consciousness. This standard definition, they argue, fails to take into account the frequent persistence of phenomenal consciousness in the absence of responsiveness. “Arguably, if one aims to explore the neural correlates of phenomenal consciousness, it would be fruitful to contrast the neural activity during dreaming anaesthesia vs. the neural activity during dreamless anaesthesia,” they said.

The study is vulnerable to some obvious criticisms. The depth of sedation was shallower than is typically used in surgery, so the results may not generalise to higher doses of anaesthesia. Also, the participants were forewarned that they would be interviewed about any experiences they had whilst unconscious, which could have led them to come up with the kind of answers that they felt the researchers were after. Defending the validity of their results, Noreika’s team pointed out that subjective reports of experience were more frequent when the objective BIS measure indicated shallower sedation – just as you’d expect if the experiences were real. “The results confirm that subjective experience may occur during clinically defined unresponsiveness,” the researchers said.

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Noreika, V., Jylhänkangas, L., Móró, L., Valli, K., Kaskinoro, K., Aantaa, R., Scheinin, H., and Revonsuo, A. (2011). Consciousness lost and found: Subjective experiences in an unresponsive state. Brain and Cognition, 77 (3), 327-334 DOI: 10.1016/j.bandc.2011.09.002

Further reading: Check out this recent New Scientist feature article on consciousness and anaesthesia.

Post written by Christian Jarrett for the BPS Research Digest.

Paraplegics walk in their dreams

In the land of dreams, the shackles of disability are cast asunder. That’s the revelation from a new dream diary study featuring 15 paraplegics, 5 of whom were born with their condition. These volunteers (aged 22 to 84 years), recruited from a military hospital and a care home for people with motor disabilities, recorded their dreams for 6 weeks and French researchers compared the content with similar diaries kept by age-matched, able-bodied control participants.

All bar one of the disabled participants had at least one dream in which they moved their legs, including all five of the congenital paraplegics. As a group, the disabled participants experienced dreams about walking twice as often as they had dreams featuring their paraplegia. Moreover, voluntary leg movements featured in the dreams of the disabled more often than in the dreams of the able-bodied (38.2 per cent of dreams vs. 28.7 per cent). “I was not in a wheelchair but walking to a night club, to go dancing,” recalled a 22-year-old person with congenital paraplegia in one typical dream report. There were some dreams featuring wheelchairs – these were experienced by eight of the disabled group and none of the controls.

Of the leg-movement mentions in the disabled participants’ dream reports, the majority (46 per cent) pertained to walking. This is a lower percentage than found in the able-bodied diaries (64 per cent), but that’s because the disabled dreamt more often about dancing and standing up. Activities like running, cycling, swimming and driving featured equally often in the dreams of both groups.

The disabled participants dreamed of walking even though they’d either never walked or hadn’t walked for years. For example, two of the participants had been paralysed since sustaining gun-shot wounds during World War II and the France-Algeria war. There was no evidence that walking dreams became less frequent with duration of paralysis.

How do people with paraplegia dream of walking if it’s something they’ve never experienced or haven’t done for years? The researchers, led by Marie-Thérèse Saurat, believe it could reflect the activity of a “genetic, inherent walking programme”, or the action of mirror neurons, which are stimulated during the day by the sight of other people performing movements. The latter explanation is especially favoured for dreams about cycling and other complex activities – it’s “difficult to imagine that there is an innate programme for riding a bicycle, as this is a highly specialised activity recently developed in human history,” the researchers said.

Why do paralysed people dream of walking? The researchers dismiss the psychoanalytic idea that the dreams are an expression of a subconscious wish. They argue that people with paraplegia are open about their desires to walk, and that their dreams are not dominated by walking to the extent you’d expect if they were compensating for lack of walking in waking life (in fact their dreams contained less walking than the control participants). Saurat and her colleagues suggest instead that walking in dreams may have an adaptive function: helping “consolidate the relevant neuronal mapping … Notably, motor imagery training improves the movement performance of the intact muscles and increases basal ganglia activation in subjects with spinal cord injury.”

These new findings add to previous research that found people born blind have visual experiences in their dreams and people born deaf can hear spoken language in theirs.
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ResearchBlogging.orgSaurat, M., Agbakou, M., Attigui, P., Golmard, J., and Arnulf, I. (2011). Walking dreams in congenital and acquired paraplegia. Consciousness and Cognition, 20 (4), 1425-1432 DOI: 10.1016/j.concog.2011.05.015

Post written by Christian Jarrett for the BPS Research Digest.

An afternoon nap tunes out negative emotions, tunes in positive ones

The perfect excuse for a siesta! People who stay awake throughout the day become progressively more sensitive to negative emotions. In contrast, those who take an afternoon nap are desensitised to negative emotions yet more responsive to positive ones. The new finding builds on past research by showing that not only does sleep deprivation cause emotional problems, a sleep boost can bring emotional advantages.

Ninad Gujar and his colleagues tested 36 participants (half were male; average age 21) on a face processing task, once at 12pm and then again at 5pm. Half the participants were given a 90-minute napping opportunity after the first task, whilst the others just went about their day as usual.

The task involved the participants looking at a computer screen that showed a male face pulling fearful, sad, angry and happy expressions at various intensities. The participants’ goal quite simply was to rate each presentation of the face for intensity on a scale from 1 (definitely neutral) to 4 (mostly happy/sad etc).

For participants who stayed awake through the afternoon, their performance at 5pm, compared with at 12pm, demonstrated heightened sensitivity to fearful and angry facial expressions. By contrast, the participants who’d had a nap were less sensitive to fearful expressions at 5pm yet more sensitive to happy expressions. These emotional processing changes were also accompanied by mood differences: the no-nap group reported less positive mood later in the afternoon, compared with earlier, whereas the nap-group reported a decrease in negative mood.

The emotional processing changes observed among the nap-group were related to rapid-eye-movement (REM) sleep. EEG recordings taken while the nappers slept showed that those who obtained REM sleep were more likely to show the desensitisation to negative emotions and sensitisation to positive ones.

An alternative interpretation of the results is that napping affects visual processing, not emotional sensitivity. But the researchers don’t think this stands up to much scrutiny, since any basic visual processing effects ought to have been uniform across the different emotions.

So, assuming the emotional sensitivity account is true, why might the non-napping participants have become more sensitive to negative emotions? One possibility, which is backed up by sleep deprivation research, is that the prefrontal cortex becomes fatigued through the day and therefore less able to dampen down emotional reactivity in the sub-cortex. Alternatively, perhaps heightened sensitivity to fear and anger is adaptive – as we fatigue through the day, it makes sense that we should become more vigilant towards these danger-based signals. Either way, a brief nap appears to give us an emotional recharge, altering the way we respond to other people’s facial expressions. The implications for working practices are obvious.

‘These data add to a growing collection of findings indicating a regulatory role for sleep in the optimal homeostasis of emotional brain function,’ the researchers said, ‘which if disrupted may have detrimental contributions to clinical symptomotology in affective disorders.’
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ResearchBlogging.orgGujar, N., McDonald, S., Nishida, M., and Walker, M. (2010). A Role for REM Sleep in Recalibrating the Sensitivity of the Human Brain to Specific Emotions. Cerebral Cortex, 21 (1), 115-123 DOI: 10.1093/cercor/bhq064

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Earlier on the Digest: How to nap

How to nap

Even naps as short as ten minutes have been shown to provide psychological benefits in terms of reduced fatigue and improved concentration (pdf). But would-be nappers face some strategic decisions, most obviously – does it matter whether I nap in my chair or ought I try to find somewhere to lie down? And then … if remaining seated, is it okay to lean forwards and rest my head on a desk?

When it comes to napping while leaning back in a chair or car seat, past research has shown that the further you can lean back, the better, at least in terms of subjective fatigue and reaction times. Now Dayong Zhao and colleagues have addressed the leaning forward issue, comparing lying-down napping and leaning-forward napping, and they’ve found that the former is the most effective, but that the leaning-forward variety still has clear benefits compared with no nap at all.

Thirty undergrads, all regular nappers, had electrodes attached to their heads before lunch. Then they performed an ‘oddball’ auditory task in which they had to listen to a string of tones and listen out for the occasional one of a different pitch. Next they had lunch before splitting into three groups: one group enjoyed a twenty minute nap lying down; another enjoyed a twenty-minute nap leaning forwards onto a desk (plus pillow for comfort); the final group just spent the same time sitting quietly.

After this, all the participants performed a repeat of the oddball task whilst having their brainwaves recorded via electroencephalography. Zhao’s team were particularly interested in the size and delay of the P300 – a brainwave measure of cognitive alertness.

Participants in both of the napping conditions showed benefits compared with their peers who’d been denied a nap. The nappers, leaning and lying, reported being in a better mood and feeling less sleepy and they performed better at the oddball task. When it came to the brainwave recordings, however, the leaning-forward nappers, unlike the lying-down nappers, showed no difference from the control group. Uniquely, the lying-down nappers showed an increased P300 amplitude, perhaps indicating increased cortical arousal on their part.

The message it seems is clear. A post-luncheon nap is beneficial to your mental functioning even if you’re forced to rest your head on your desk. However, if you can find somewhere to lie down properly, then do, because the benefits of the nap will be that much greater.
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ResearchBlogging.orgZhao, D., Zhang, Q., Fu, M., Tang, Y., & Zhao, Y. (2010). Effects of physical positions on sleep architectures and post-nap functions among habitual nappers. Biological Psychology, 83 (3), 207-213 DOI: 10.1016/j.biopsycho.2009.12.008

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Scientists find way to strengthen memories during sleep

If only we could make more constructive use of all the time that we spend asleep. People have tried playing various tapes to themselves while they’re dozing, from foreign vocab lists to stop-smoking mantras, but they’re all the wrong side of useless. What we do know for sure is that sleep is important for memory consolidation, if only we could tap into this somehow. Now, finally, John Rudoy and colleagues have provided some elusive evidence for how learning during sleep can be enhanced.

Twelve participants looked on as fifty objects appeared one at a time in various locations on a computer screen. Importantly, as each object appeared it was accompanied by a characteristic noise – for example a cat appeared with a meow and a kettle with a whistle. Several rounds of learning took place until the participants had estimated the approximate location of each object at least once. A final pre-nap test was then performed so that the researchers knew how well participants knew each object location before they went to sleep.

That the participants had nodded off was confirmed with brain wave recordings via scalp electrodes. But here’s the clever bit. As the participants dozed off into non-REM slow-wave sleep, the researchers played the sounds associated with 25 of the objects. The objects that were cued in this way were carefully chosen such that pre-nap memory performance had been equal for cued and un-cued objects.

The participants woke up after about an hour and the exciting finding is that although their overall memory accuracy was lower compared with before the nap, their performance for the objects cued whilst they slept was superior to un-cued objects, even though pre-nap performance for the two object groups had been equal.

The researchers also looked back at the brain wave signals recorded during sleep, comparing the brain’s response to sounds associated with objects that were better remembered on waking relative to objects for which memory had deteriorated. They found the brain had responded more to sounds belonging to better remembered objects. “We propose that sound cues presented during sleep prompted preferential processing of corresponding object-location associations,” the researchers said.

For sceptics who think the results may have nothing to do with sleep, the researchers repeated the noise cueing exercise with twelve participants who remained awake. In their case, sounds presented after learning made no difference to subsequent memory performance.
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ResearchBlogging.orgRudoy, J., Voss, J., Westerberg, C., & Paller, K. (2009). Strengthening Individual Memories by Reactivating Them During Sleep. Science, 326 (5956), 1079-1079 DOI: 10.1126/science.1179013

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Slumber quality important for learning

It’s not just the amount of sleep we get that is so important for learning, but the quality of that sleep. That’s according to a new study that made precise use of beeping noises to disrupt deep “slow-wave” sleep among 13 elderly participants (average age 60 years), without actually waking them up.

The beeping was used in such a way that although the participants’ were deprived of deep sleep, their total sleep time and number of sleep stages were unaffected (compared with a comparison night of undisturbed sleep).

After a night of either shallow or deep sleep, the participants had their brains scanned while they viewed 50 images of houses and landscapes. The next day they had to say which of 100 images were repeated from the day before. The participants’ performance was superior when a night with deep sleep had preceded the learning of the images, compared with a night of shallow sleep, even though total sleep time was the same in each case (36.6 images correctly identified versus 31.4 images, on average).

Moreover, the brain scans showed that during the initial viewing of images, activity in the hippocampus, the seat of human memory, was reduced after shallow versus deep sleep, but only for those images that were subsequently recalled. This suggests that shallow sleep somehow interferes with the way the hippocampus encodes new, explicit memories.

By contrast, so-called “implicit memory”, appears to be unaffected by sleep quality. Regardless of the kind of sleep they’d had, participants showed superior performance at a sequence learning task when the sequence was fixed rather than random, even though they were consciously unaware of what the actual sequence was.

“The mechanism by which deep sleep affects hippocampal function is unclear,” Ysbrand Van Der Werf and colleagues said, “but may involve local synaptic changes resulting from slow wave activity.”
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ResearchBlogging.orgYsbrand D Van Der Werf, Ellemarije Altena, Menno M Schoonheim, Ernesto J Sanz-Arigita, Jose´ C Vis, Wim De Rijke, Eus J W Van Someren (2009). Sleep benefits subsequent hippocampal functioning. Nature Neuroscience. In Press.

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Older people have more black and white dreams

If you dream in colour, you’re not alone: the majority of people today claim to have colourful dreams. But it wasn’t always thus. Research conducted in the early part of the last century consistently found that people reported dreaming most often in black and white.

According to Eva Murzyn at the University of Dundee, there are at least two possible explanations for this strange anomaly.

The first is methodological. The early studies tended to use questionnaires, whereas more modern studies use dream diaries (filled in upon rising in the morning) or so-called “REM-awakening”, which involves interrupting people’s dream-filled periods of sleep to find out what they were dreaming about. People’s memories of their dreams are likely to be less accurate using the questionnaire approach and more likely to reflect lay beliefs about the form dreams generally take.

The second explanation has to do with black and white television and film. It’s possible that the boom in black and white film and television during the first half of the last century either affected the form of people’s dreams at that time, or affected their beliefs about the form dreams generally take.

According to Murzyn’s findings, it’s the explanation based on media exposure that carries more weight. She used both questionnaire and diary methods to study the dreams of 30 older (average age 64) and 30 younger people (average age 21).

The methodological technique made no difference to the type of dreams people reported. Crucially, however, across both questionnaires and diaries, the older participants (who had had significant early life exposure to black and white media) reported experiencing significantly more black and white dreams over the last ten days than the younger participants (22 per cent vs. 4 per cent).

Another finding was that older participants reported black and white dreams and colour dreams to be of equal vividness. By contrast, the younger participants reported that the quality of black and white dreams was poorer. This raises the possibility that the younger participants didn’t really have any black and white dreams at all, but were simply labelling poorly remembered dreams as black and white.

Several awkward questions are left unanswered by this study. It’s not clear if the older participants really are experiencing more black and white dreams or if it’s their memories or beliefs about dreams that is influencing their reports. Related to this, we don’t know if early exposure to black and white media has really affected the form of the older participants’ dreams or simply their beliefs about dreams. Finally, if differences in media exposure really do explain the current results, we’re still left with the question of how and why early exposure to black and white TV and film has had such an effect on the older participants, even after so many years of exposure to colour media and given that they live every day in a colourful world.
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ResearchBlogging.orgE MURZYN (2008). Do we only dream in colour? A comparison of reported dream colour in younger and older adults with different experiences of black and white media Consciousness and Cognition. DOI: 10.1016/j.concog.2008.09.002

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Goodbye insomnia, hello warming body-suit

Forget counting sheep or popping pills, a team of Dutch researchers have reported the profound sleep-inducing effect of a warming body-suit.

Eight young adults and sixteen older adults, half of whom suffer from insomnia, spent two nights in a body-suit at a sleep laboratory (see image), with a night at home in between.

Water-filled micro-pipes in the suit maintained the skin temperature of the participants at either 35 degrees celsius in the cool condition or 35.4 degrees in the warm condition, fluctuating gradually between the two every 15 to 30 minutes. Importantly, core body temperature was unaffected by these subtle temperature fluctuations.

The controlled skin temperatures match the typical climate of a person’s bed and are close to the levels that people report to be of most comfort, with the warmer condition actually reported to be slightly less comfortable.

Recordings of the participants’ brain waves at night showed that warmer skin temperatures resulted in a shift in sleep depth towards deeper sleep and a reduction of their likelihood of being awake at 6am.

For instance, among the non-insomniac older participants, a subtle (only 0.4 degree) increase in skin temperature reduced the probability of being awake at 6am by a factor of 14; for those with a sleep problem, it was by a factor of five. Moreover, with the same subtle increase in temperature, the likelihood of an older insomniac participant being in a deep (slow wave) sleep was doubled for any point in the night.

Roy Raymann and colleagues who conducted the research believe skin temperature affects cells in the hypothalamus of the brain that are responsible for controlling sleep.

The findings have huge practical implications, even before the development of user-friendly body-suits. For example, it is possible that the temperature environment people choose to sleep in, based on comfort, may not be optimal for inducing sleep.

A warm bath before bedtime could help increase skin temperature at the start of the night, and a timed electric blanket could be used to increase skin temperature in the morning. Thick blankets or an all-night electric blanket won’t help because they will simply cause overheating, especially of core body temperature, which will disrupt sleep.

“The effects of even very minimal temperature manipulations within the thermoneutral comfortable range are so pronounced that they warrant further research into practical thermal manipulation applications to improve sleep,” the researchers concluded.
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Raymann, R.J., Swaab, D.F., Van Someren, E.J. (2008). Skin deep: enhanced sleep depth by cutaneous temperature manipulation. Brain, 131(2), 500-513. DOI: 10.1093/brain/awm315

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Our bodies fail to adjust to seasonal clock changes

The UK and other countries have been altering their clocks with the seasons since at least the beginning of the last century.

The argument in favour of Daylight Saving Time (DST), or British Summer Time as it’s called in the UK, is clear: daylight hours are effectively shifted from the early morning to the evening, so that there is more time for sport and leisure in the summer months.

Increasingly, however, there have been calls for Britain to remain on British Summer Time throughout the year – to stay coordinated with Europe, and to reduce energy consumption in the dark winter months. Now Thomas Kantermann and colleagues have added to the debate with a study showing that the ability of our bodies to acclimatise to the natural changes in sunrise onset is lost following the artificial shift of the clocks forward each Spring.

Kantermann’s team took advantage of self-report data from more than 55,000 people gathered as part of the Munich Chronotype Questionnaire. This showed that the timing of people’s sleeping and waking tracks the natural changes to dawn time that occur during the Winter months (i.e. during standard time) but fails to show the same adjustment after the introduction of Daylight Saving Time in the Spring.

In another study, the researchers used actimeters (gadgets strapped to the wrist that measure movement) to track the sleep patterns of 55 volunteers for four weeks prior to, and four weeks after, the clock changes in the Autumn and Spring of 2006-2007. Again, this confirmed that after the advancement of clocks in the Spring, people’s sleep patterns failed to adjust to the seasonal shifting of sunrise, as they had been doing before the clock change. This was especially the case for ‘owls’ – people with a preference for the evening who find it difficult to get up in the morning.

These results reflect the way people suffer more from jet leg when travelling westwards and time is advanced, compared with travelling eastwards, ‘backwards’ in time.

Co-author Till Roenneberg said: “It is much too early to say whether DST has a serious long-term impact on health, but our results indicate that we should consider this seriously and do a lot more research on the phenomenon.”
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Kantermann, T., Juda, M., Merrow, M. & Roenneberg, T. (2007). The human circadian clock’s seasonal adjustment is disrupted by daylight saving time. Current Biology, 17, 1996-2000.

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Students: it’s time to ditch the pre-exam all-nighter

Lack of sleep impairs the human brain’s ability to store new information in memory, researchers have found. Past research has already shown that sleep is vital for consolidating recently-learned material but now Matthew Walker and colleagues have shown that sleep prior to learning is just as important.

“The implications of our findings have never been more relevant than in the present day, when sleep hygiene and total sleep time are declining across all age ranges”, the researchers said, pointing to the “all-nighter” before exams as the “quintessential example” of how we deliberately deprive ourselves of sleep.

The researchers scanned the brains of 28 participants who attempted to remember a series of pictures of people, landscapes, scenes and objects. Half the participants had slept the previous night as usual and acted as controls; the other participants had been kept awake, meaning they’d gone about 36 hours without sleep (the learning task was in the evening). Two days later, after everyone had two nights of normal sleep, the participants were shown more pictures and asked to identify those they’d been shown earlier in the week. Compared with the controls, the previously sleep deprived participants recognised 19 per cent fewer pictures.

Brain scans taken at the time the participants learned the pictures provided some clues as to how sleep deprivation had affected learning-related activity in the brain. Compared with the controls, the sleep deprived participants showed less activity in the hippocampus, the brain area associated with the laying down of episodic memories. Moreover, whereas the hippocampi of the controls was functionally connected with the frontal and parietal lobes, in the sleep deprived participants it was connected with basic alertness networks in the brain-stem – what the researchers said was “potentially a cooperative mechanism attempting to elevate levels of alertness during memory encoding”.

Why does lack of sleep have this effect on memory? The researchers aren’t sure, but it’s possible that lack of sleep denies the brain the opportunity to transfer episodic memories in the hippocampus into long term storage, so that the capacity of the hippocampus effectively becomes filled the longer we’re awake.
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Jyoo, S-S., Hu, P.T., Gujar, N., Jolesz, F.A. & Walker, M.P. (2007). A deficit in the ability to form new human memories without sleep. Nature Neuroscience, Advance Online Publication.

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

Link to related Digest item.