Category Archives: Dreaming

Sleep’s Positive Impact on Traumatic Memories


Sleep is inescapable. Some will see it as a necessary evil and others will claw at it with limited success. It may leave us (largely) immobile but it is anything but a passive state. Sleep paves the way for new neural connections to be made, clears the brain of harmful waste products, is protective against mental and physical illness, and is preferable to leaving the bed on a cold Monday morning. There is still plenty of debate as to the true function of sleep, but one key area which it appears to be important for is memory.

Broadly speaking, it is argued that sleep enhances the consolidation of recently learned information compared to wakefulness. For example, a nap as short as 5-10 minutes has been shown to enhance memory and improve problem solving compared to wakefulness following learning. Moreover, rapid eye movement sleep has been linked to enhanced memory for emotional information, and to reducing the emotional strength of such memories. This suggests that different aspects of sleep are important for different types of memories and highlights some potential ways we can modify the impact of negative ones. What if we could interfere with the brain’s ability to form negative memories? More specifically, what if we could interfere with traumatic and intrusive memories which form the basis of illnesses such as post-traumatic stress disorder (PTSD)? Recent research seems to suggest this may be possible.

PTSD is a psychiatric disorder characterised by flashbacks and nightmares, avoidance of the situation in which the trauma occurred, and emotional numbing. It is the result of a traumatic event which can be wide in scope (e.g. warfare, sexual abuse, hospital admission) and individuals with PTSD will often find themselves transported back to the event. These ‘memories’ are intrusive in nature and, understandably, highly distressing. It has been suggested that interfering with the consolidation of intrusive memories at the time they are formed (or shortly afterwards) may help those who are likely to go on to experience PTSD. Evidence to support this would unlock a powerful early intervention tool for those likely to experience trauma (e.g. refugees or soldiers). One way this can be achieved, as you have likely guessed, is through manipulating sleep.

In 2015, a study conducted at the University of Oxford explored whether sleep deprivation might be protective against intrusive traumatic memories. Participants were brought into the laboratory and randomised to either a sleep deprivation or sleep group. They were then shown a film containing traumatic scenes which lasted just over 15 minutes. Following the film, participants were either kept awake or allowed to sleep. On the following day, participants completed a questionnaire measure of PTSD symptomology and were then asked to complete a diary to track the intrusiveness, content, distress, and presence of mental imagery associated with the traumatic film for 6 days.

The Oxford group, led by Dr. Kate Porcheret, found that a night of sleep deprivation, compared to sleep as normal, reduced the intrusiveness of experimental-trauma memories. The sleep deprivation group reported fewer intrusive memories, lower PTSD symptoms scores, and (non-significantly) reduced distress of the intrusive memories. However, this effect was only found for the first two days and sleep deprivation was found to confer no protection against intrusive memories after 6 days. This suggests that sleep deprivation immediately after the traumatic experience can reduce its intrusiveness but only in the short term. Sorted. Sleep deprivation, paradoxically, confers some protective against traumatic memories. Right? Well, to quote Ben Goldacre, “I think you’ll find it’s a bit more complicated than that”.

A study published last December in the aptly named journal SLEEP has injected additional complexity to this issue. Dr. Birgit Kleim and colleagues assessed the impact of a single night of sleep deprivation on distress and intrusiveness of traumatic memories for seven, rather than six, days. They asked 65 female participants to watch two 12 minute films of a neutral (nature documentary) or traumatic (horror film) nature in a randomised order. Participants were either allowed to sleep at home or kept awake. They were then asked to keep an intrusion diary which required participants to note when they experienced intrusive memories related to the film and rate their vividness, intrusiveness, content, and distress each day. The participants were then followed-up a week later.

So, what did they find? For the first two days following the films, there was no significant difference between the sleep or wake groups on distress or intrusiveness of the traumatic film. However, by days 6 and 7 there was a significant reduction in distress and intrusiveness for the sleep compared to wake group. By contrast, there were no difference in intrusiveness or distress of the neutral film for the sleep and wake groups. The effects found were specific for the traumatic, emotional, memory.

This second study showed that sleep deprivation does not provide a protective effect against intrusive emotional memories. Instead, they argued that sleep immediately following the trauma experience has long-term benefits on reducing the distress and intrusions associated with the traumatic memory. Nonetheless, this study does not directly contradict the one conducted by Porcheret and colleagues at Oxford. Although non-significant, distress was higher for the sleep compared to sleep deprivation group which suggests that sleep deprivation may serve an immediate protective role against traumatic memories. However, this effect seems to reverse in the relative long-term.

Why should this be so? Well, we know that memories – particularly emotional ones – are strengthened by a period of sleep. This would suggest that sleep following a traumatic experience would strengthen the memory for that experience and thus enhance the intrusiveness of a traumatic memory. This could explain why sleep deprivation produced a reduction in intrusiveness and distress for the Oxford study as the lack of sleep interfered with the ability to lay down the negative memory. However, the explanation for these studies is less clear. The authors argue that sleep deprivation is protective against intrusive memories in the short term but not the long-term. Kleim and colleagues claim that sleep following the traumatic experience may initially make it more distressing but also encourage appropriate integration of the memory alongside existing memories. This, they argue, reduces the chance that the traumatic memory will be intrusive and uncontrollable – a cardinal symptom of PTSD.

We already know that rapid eye movement sleep (REM) is associated with a reduction in the intensity of emotional images (van der Helm et al., 2011). Kleim’s study found that increased REM was associated with more, not less, intrusions. Instead, they argue that other stages of sleep are responsible for the reduction in distress and intrusions seen perhaps through a different mechanism. However, this does not state that the memory should be remembered more poorly (i.e. deliberate recall of the film would be unaffected). Rather it seems that deliberate memory recall and intrusive memories may be guided by different mechanisms and differentially affected by sleep. Neither the Porcheret or Kleim study asked participants to take a memory test of their explicit recall of the films. However, the diary studies suggest that all participants were accurate in recalling the films throughout the study period and deliberate recall does not appear to be associated with intrusive memory frequency.


A similar distinction between intrusive and deliberate recall of memories was found by another study attempting to reduce the negative impact of traumatic memories. Indeed, these are the not the first studies which have attempted to interfere with the consolidation of traumatic memories. A more colourful way of achieving this has been shown through getting people to play Tetris. For the uninitiated, Tetris is a simple game whereby you match coloured bricks of different shapes into lines of 4. They fall from the top of the screen and you have to rotate the shapes to make them line-up and disappear. In 2009, Dr Emily Holmes and colleagues at the University of Oxford showed that if they got participants to play Tetris for 10 minutes, half an hour after a traumatic film, they saw a reduction in subsequent intrusions or “flashbacks”. This effect was found for over a week follow-up during which an intrusion diary was kept. Interestingly, deliberate recall of the film was not impaired when tested at one-week follow-up. This also suggests it is possible to reduce the intrusive nature of a traumatic memory without reducing the memory for the event.

Of course, it is difficult to get someone in a warzone to take out their Tetris ration or take a nap following a fire-fight, but it highlights that it is possible to reduce the negative impact of traumatic memories. These pieces of research suggest that manipulating sleep is a viable way to reduce the ‘flashback’ quality of traumatic memories. Although promoting sleep for those having recently experienced a traumatic episode might raise its own difficulties, it heralds a step towards early intervention for PTSD. If nothing else, they remind us that sleep is important for the consolidation of memory alongside existing memory networks and how little we know about the effect of sleep on memory.

One thing we do know for sure: sleep is anything but a passive and simple state.

Inquisitive Tortoise


Holmes, E. A., James, E. L., Coode-Bate, T., & Deeprose, C. (2009). Can playing the computer game “Tetris” reduce the build-up of flashbacks for trauma? A proposal from cognitive science. PloS one, 4(1), e4153.

Kleim, B., Wysokowsky, J., Schmid, N., Seifritz, E., & Rasch, B. (2016). Effects of Sleep After Experimental Trauma on Intrusive Emotional Memories. Sleep.

Porcheret, K., Holmes, E. A., Goodwin, G. M., Foster, R. G., & Wulff, K. (2015). Psychological effect of an analogue traumatic event reduced by sleep deprivation. SLEEP, 38(7).

van der Helm, E., Yao, J., Dutt, S., Rao, V., Saletin, J. M., & Walker, M. P. (2011). REM sleep depotentiates amygdala activity to previous emotional experiences. Current Biology, 21(23), 2029-2032.

Image Credits:

Bad Memory (Header)

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Filed under Dreaming, Memory, Psychology, Sleep Science, Trauma

Why Do We Dream?


Key Dream Research Equipment

“What does my dream mean?!”


“I’ve heard you study sleep. I mean I think I’ve worked it out myself but I wanted an expert opinion.”


It doesn’t take long to drift into questions about dream analysis when I tell people that I study sleep. I don’t entirely blame them. It’s a subject which has fascinated society for millennia and it doesn’t look like that is going to stop any time soon. I’m sure most of us have had a dream which we are convinced has some greater significance: a dream about facing our fears or that person who you hadn’t thought about for years.

In the distant past, dreams were associated with divine will and prophetic qualities. Ancient Egyptian and Greek scholars produced dream manuals which were used to interpret nightly visions. Flash forward to the 19th and early 20th century and there was a shift towards understanding dreams in terms of their psychological causes and consequences. It was not until 1953 with the discovery of rapid eye movement (REM) sleep that the objective study of dreaming was possible.

Since then, we have come to appreciate that not all dreams are not created equal. Depending on which stage of sleep the dream is recalled from will depend on the type of dream. Firstly, dreaming does not only occur during rapid eye movement (REM) sleep but can occur at any stage during the sleep cycle. Dreams at sleep onset and during short naps appear to be relatively faithful, if not stripped back, representations of daily activities. By contrast, dreams reported during REM sleep and after longer periods of sleep appear more bizarre and less clearly linked to daytime activities.

So why do we dream? What attempts have there been to try to understand these nightly visions of the surreal?

Wish Fulfilment

Freud stated in ‘The Interpretation of Dreams’ that all dreams were wish fulfilment of our uninhibited desire. The dream provided an outlet for these desires and prevented them seeping into wakefulness. The dream (manifest) content could thus be interpreted and inform the analyst about the unconscious desires of an individual. The latent, or unremembered and unconscious, dream content could be reached through employing dream analysis and psychoanalytic techniques. Although you can still buy manuals which claim to teach you how to analyse dreams in line with early psychoanalytical thought, the validity of these are questionable at best. The steps required to reach the ‘latent content’ can be idiomatic and despite claims that an airplane can represent unconscious desires sometimes “…a cigar is just a cigar”.

Dreaming to Remember

It has been shown that dreaming is linked to the activities which we complete during the day. This should come with little surprise to most. We can often pinpoint our dreams back to activities we have completed during the day – even if they appear in bizarre contexts.

Robert Stickgold at the Department of Psychiatry, University of Harvard, showed this with a simple experiment. Participants were required to play Tetris at fixed periods and to sleep in a monitored laboratory setting. During this time, participants were asked about their thoughts and dreams in the first hour of attempted sleep.  He found that by asking participants to play Tetris for extended period of time that a significant number of sleep-onset dreams and images were found to be linked to Tetris (i.e. images of the blocks falling into place and lines of blocks disappearing when complete).


A study from the same group led by Erin Wamsley at the University of Harvard built on this by exploring whether the presence of task-relevant dreams was associated with performance. They asked participants to complete a virtual navigation task whereby they had to reach a goal (e.g. a tree). Participants were then re-tested just over 4 hours after the initial completion of this task.

Half of the group were then provided with an opportunity to nap and the other half remained awake. Unsurprisingly, from what we already know, the nap group were found to show a greater improvement on the task at re-test. Interestingly, and importantly for dreaming, the participants who dreamed about the maze during the nap also reported the greatest re-test improvement. Okay, perhaps this might simply have been due to the fact that these participants were simply thinking of the task more? However, this doesn’t appear to be the case. The wake participants were also asked about their task-relevant thoughts but the researchers found no benefit to re-test performance here.

Collectively, these studies suggest that dreaming may be a result of the brain consolidating and organising new memories alongside existing ones. However, they don’t tell us that dreaming is responsible for our improvement but that dreaming may be a marker of it.

Activation Synthesis Theory (and Dreaming to Forget)

Other theories suggest that dreaming has little relevance to memory or other functions. The Activation Synthesis theory proposed by Allan Hobson and Robert McCarley in 1977 claims that dreaming is the product of seemingly random or automatic brain activity produced during REM sleep.

These ‘random’ activations have their origin in the brain stem and are transmitted to the forebrain during REM sleep. The cortex then compares this activation against previous memories to make sense of the neural activity produced by the brain stem. It is claimed that a similar mechanism may explain non-sleep hallucinations and dreaming. This theory forms the basis for another hypothesis for why we dream: not to remember but to forget. This should seem counter-intuitive based on what we know about sleep and memory – and all that advice you’re given by teachers or colleagues before an important exam or meeting.

However, Francis Crick (yes, that Francis Crick) and Graeme Mitchison argued in 1983 that sleep has an important role in identifying faulty connections within the brain – an inevitable problem facing a neural network of sufficient complexity. These ‘faults’ arise during development and as a part of the multitudinous memories and connections we develop throughout any ordinary day. During REM sleep, these ‘faults’ are dampened down and their connections weakened. This ensures the brain works efficiently.

So, where does dreaming fit in?

Quite simply the ‘faults’ in the brain are experienced as dreams. Our dreams are the connections which our brains’ are trying to suppress during REM sleep. The overall purpose of such a mechanism is to ensure the efficient functioning of a brain with finite processing power.

However, if our dreams are the memories we are trying to erase, in one form or another, why do we have recurrent dreams? Crick claims that this is tricky for the theory but may be as a result of the threatening nature of recurrent dreams. The anxiety and fear associated with such dreams is likely to wake up the dreamer and to subsequently be remembered. This may interfere with the reverse learning process.

Primitive instinct rehearsal theory of dreaming

Another appealing theory is that dreaming is not a product of vital or random brain activity but rather that it serves an evolutionary advantage. More specifically, it has been claimed that dreams may help us deal with threatening situations in our daily lives by giving us an opportunity to practise overcoming them. In turn this increases our survival odds and also increases the chance we will successfully reproduce – passing on that ability to dream to our offspring.

Anecdotally this might explain why anxiety and fearful dreams are commonly reported by many different people, and why they share a common theme. However, like Freud, this theory assumes that dream content must have a distinct purpose. It’s hard to falsify such a theory and provide evidence which would conclusively disprove its existence as many dreams could conceivably be interpreted in a practise or threat-related manner.

Dreaming and Creativity

Finally, it’s an old claim that creativity and dreaming are linked together. Authors such as Mary Shelley and Steven King relate their tales of horror back to dreams they’ve remembered. Salvador Dali enjoyed the phantasmagorical images produced through his dreams and used them as inspiration for his surrealist art.

What does the science say about creativity and dreaming?


Surveys of students’ ability to remember and recall dreams correlates with openness to new experiences, fantasy proneness, and may even be associated with a lesser ability to filter out environmental input. Cumulatively, this suggests that traits associated with creativity and ‘thinking outside the box’ are associated with an ability to recall dreams. However, this does not mean that we dream in order to improve creativity but that creativity is likely a product of bizarre, frightening and fantastical dreams. We benefit from dreaming but only insomuch as an incidental source of inspiration.

Overall, even though the content of your dreams may not necessarily provide a convenient road to your hidden thoughts and desires, they can provide us some fascinating insights into the sleeping brain. Despite frustrated responses from others, that remains my response when they ask about last night’s dream about planes, trains and giant chickens.

Don’t ask.

Inquisitive Tortoise


Crick, F., & Mitchison, G. (1983). The function of dream sleep. Nature, 304(5922), 111-114.

Stickgold, R., Malia, A., Maguire, D., Roddenberry, D., & O’Connor, M. (2000). Replaying the game: hypnagogic images in normals and amnesics. Science, 290(5490), 350-353.

Wamsley, E. J., & Stickgold, R. (2010). Dreaming and offline memory processing. Current Biology, 20(23), R1010-R1013.

Wamsley, E. J., Perry, K., Djonlagic, I., Reaven, L. B., & Stickgold, R. (2010). Cognitive replay of visuomotor learning at sleep onset: temporal dynamics and relationship to task performance. Sleep, 33(1), 59-68.

Image Credits:

Dream Catcher (Header)

Tetris (Body Text)

Nightmare (Body Text)


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