Sleep

How sleep acts on the brain

Sleep reorganizes your memories

December, 2010

New studies show how sleep sculpts your memories, emphasizing what’s important and connecting it to other memories in your brain.

The role of sleep in consolidating memory is now well-established, but recent research suggests that sleep also reorganizes memories, picking out the emotional details and reconfiguring the memories to help you produce new and creative ideas. In an experiment in which participants were shown scenes of negative or neutral objects at either 9am or 9pm and tested 12 hours later, those tested on the same day tended to forget the negative scenes entirely, while those who had a night’s sleep tended to remember the negative objects but not their neutral backgrounds.

Follow-up experiments showed the same selective consolidation of emotional elements to a lesser degree after a 90-minute daytime nap, and to a greater degree after a 24-hour or even several-month delay (as long as sleep directly followed encoding).

These findings suggest that processes that occur during sleep increase the likelihood that our emotional responses to experiences will become central to our memories of them. Moreover, additional nights of sleep may continue to modify the memory.

In a different approach, another recent study has found that when volunteers were taught new words in the evening, then tested immediately, before spending the night in the sleep lab and being retested in the morning, they could remember more words in the morning than they did immediately after learning them, and they could recognize them faster. In comparison, a control group who were trained in the morning and re-tested in the evening showed no such improvement on the second test.

Deep sleep (slow-wave sleep) rather than rapid eye movement (REM) sleep or light sleep appeared to be the important phase for strengthening the new memories. Moreover, those who experienced more sleep spindles overnight were more successful in connecting the new words to the rest of the words in their mental lexicon, suggesting that the new words were communicated from the hippocampus to the neocortex during sleep. Sleep spindles are brief but intense bursts of brain activity that reflect information transfer between the hippocampus and the neocortex.

The findings confirm the role of sleep in reorganizing new memories, and demonstrate the importance of spindle activity in the process.

Taken together, these studies point to sleep being more important to memory than has been thought. The past decade has seen a wealth of studies establishing the role of sleep in consolidating procedural (skill) memory, but these findings demonstrate a deeper, wider, and more ongoing process. The findings also emphasize the malleability of memory, and the extent to which they are constructed (not copied) and reconstructed.

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Why quiet time is important for learning & memory

January, 2010

As well as during sleep, it now appears that restful periods while you are awake are also times when consolidation can occur.

It is now well established that memories are consolidated during sleep. Now a new study has found that restful periods while you are awake are also times when consolidation can occur. The imaging study revealed that during resting (allowed to think about anything), there was correlated activity between the hippocampus and part of the lateral occipital complex. This activity was associated with improved memory for the previous experience. Moreover, the degree of activity correlated with how well it was remembered. You can watch a 4 ½ minute video where the researchers explain their study at http://www.cell.com/neuron/abstract/S0896-6273%2810%2900006-1

Reference: 

Tambini, A., Ketz, N. & Davach, L. 2010. Enhanced Brain Correlations during Rest Are Related to Memory for Recent Experiences. Neuron, 65 (2), 280-290.

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A midday nap markedly boosts the brain's learning capacity

February, 2010

Students given a 90-minute nap in the early afternoon, after a rigorous learning task, did markedly better at a later round of learning exercises, compared to those who remained awake throughout the day.

Following on from research showing that pulling an all-nighter decreases the ability to cram in new facts by nearly 40%, a study involving 39 young adults has found that those given a 90-minute nap in the early afternoon, after being subjected to a rigorous learning task, did markedly better at a later round of learning exercises, compared to those who remained awake throughout the day. The former group actually improved in their capacity to learn, while the latter became worse at learning. The findings reinforce the hypothesis that sleep is needed to clear the brain's short-term memory storage and make room for new information. Moreover, this refreshing of memory capacity was related to Stage 2 non-REM sleep (an intermediate stage between deep sleep and the REM dream stage).

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The preliminary findings were presented February 21, at the annual meeting of the American Association of the Advancement of Science (AAAS) in San Diego, Calif.

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Sleep helps consolidation of a complex motor-learning task

July, 2010

Another study demonstrating the benefits of sleep for learning motor skills (in this case, a popular video game called "Guitar Hero III".

A number of studies have shown the benefits of sleep for consolidating motor learning. A new study extends this research to a more complex motor task: "Guitar Hero III", a popular video game. There was significantly greater improvement after a night’s sleep (average 68% in performance accuracy vs 63% for students who learnt the task in the morning and were tested in the evening), and a significant correlation between sleep duration and the amount of improvement.

Reference: 

Higginson, C.D. et al. 2010. So you wanna be a rock star? Sleep on it. Presented at SLEEP 2010, the 24th annual meeting of the Associated Professional Sleep Societies LLC, in San Antonio, Texas.

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Dreams are the brain's way of communicating important memory functions

April, 2010

It’s now well established that sleep plays an important role in memory and learning. Now a new study suggests that dreams also play a part in consolidating memories — perhaps reflecting the brain's attempt to find useful associations.

It’s now well established that sleep plays an important role in memory and learning. Now a new study suggests that dreams also play a part in consolidating memories. The study involved 99 subjects training for an hour on a computerized maze task, and then either taking a 90-minute nap or engaging in quiet activities. Intermittently, subjects were asked to describe what was going through their minds, or what they had been dreaming about. Five hours after training, the subjects were retested on the maze task. While those who hadn’t slept showed no improvement on the second test (even if they had reported thinking about the maze during their rest period), and those nappers who reported no maze-related dreams also showed little improvement, those who dreamed about the task showed dramatic improvement. Those who dreamed about the task were not more interested or motivated, but they were more likely to have performed relatively poorly during training — suggesting that the sleeping brain is more likely to focus on areas of greatest need. The researchers believe not that dreaming causes you to remember, but that dreaming is a marker that the brain is working on a problem at many levels — perhaps reflecting the brain's attempt to find useful associations.

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Sleep loss and temporal memory

Journal Article: 

Harrison, Yvonne & Horne, James A. 2000. Sleep loss and temporal memory. The Quarterly Journal of Experimental Psychology, 53A (1), 271-279.

Recognition memory for faces was unaffected by being deprived of sleep for 35 hours.

However, sleep-deprived subjects were significantly worse in remembering in which of two sets of photos particular faces had appeared in.

Sleep-deprived subjects who had been given significant doses of caffeine remembered the set better than those who had not, but were still poorer at remembering than those not deprived of sleep.

Although their performance was poorer, sleep-deprivation seemed to increase the subjects' belief in their own accuracy.

It seems likely that sleep-deprivation affects memory for context.

In this study, subjects were shown two sets of 12 color photographs of people’s faces (24 in total). Five minutes after seeing the last one, the subjects were then shown another 48 faces (one by one, as before) and had to say whether or not they had seen the face earlier. If so, they were asked whether they saw it in the first or second set of photographs. Half the subjects had been deprived of sleep for the previous 35 hours. Some of these had been given significant amounts of caffeine to offset their sleepiness.

It was found that the sleep-deprived subjects, whether or not they had had caffeine, were as good as the non-sleep-deprived subjects at recognizing which faces they had seen before. However, the sleep-deprived subjects were significantly worse at remembering which set the faces had appeared in. This occurred even though otherwise optimum conditions for recall existed (the test was novel, stimulating, and relatively short; it was given at the best time of day for maximum alertness).

Caffeine significantly reduced the feelings of sleepiness and did appear to improve the ability of the sleep-deprived subjects to remember which set the face had appeared in, but the level of recall was still significantly below the level of the non-sleep-deprived subjects. Caffeine made no difference to the memory performance of subjects who were not sleep-deprived.

Interestingly, sleep deprivation increased the subjects’ belief that they were right, especially when they were wrong. In this case, whether or not they had had caffeine made no difference.

It may be that the problem with temporal memory reflects a more general problem with remembering context information.

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