Friday, April 23, 2010


Why do we need sleep?
A lot of research and theories have been thrown up over the years. Scientists have shown numerous ways in which sleep is related to memory. Working memory was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic memory. As the field of sleep research is still relatively new, scientists have yet to determine exactly why people sleep. However, they do know that humans must sleep and, in fact, people can survive longer without food than without sleep. And people are not alone in this need – all mammals, reptiles and birds sleep.
Scientists have proposed the following theories on why humans require sleep:
• Sleep may be a way of recharging the brain. The brain has a chance to shut down and repair neurons and to exercise important neuronal connections that might otherwise deteriorate due to lack of activity.
• Sleep gives the brain an opportunity to reorganize data to help find a solution to problem, process newly learned information and organize and archive memories.
• Sleep lowers a person’s metabolic rate and energy consumption.
• The cardiovascular system also gets a break during sleep.
• During sleep, the body has a chance to replace chemicals and repair muscles, other tissues and aging or dead cells.
• In children and young adults, growth hormones are released during deep sleep.
• When a person falls asleep and wakes up is largely determined by his or her circadian rhythm, a day-night cycle of about 24 hours. Circadian rhythms greatly influence the timing, amount and quality of sleep. In these respects sleep conserves much energy in such mammals, particularly as sleep can also develop into a torpor, whereby metabolic rate drops significantly for a few hours during the sleep period. On the other hand, humans can usually rest and relax quite adequately during wakefulness, and there is only a modest further energy saving to be gained by sleeping.
Certainly a number of studies have shown that animals and humans deprived of sleep do not perform well on memory tasks.
The effect of sleep on memory and learning
Some memory tasks are more affected be sleep deprivation than others. A recent study, for example, found that recognition memory for faces was unaffected by people being deprived of sleep for 35 hours. However, while the sleep-deprived people remembered that the faces were familiar, they did have much more difficulty remembering in which of two sets of photos the faces had appeared. In other words, their memory for the context of the faces was significantly worse.
While large doses of caffeine reduced the feelings of sleepiness and improved the ability of the sleep-deprived person
These results indicate that sleep deprivation affects different cognitive tasks in different ways, and also that parts of the brain are able to at least partially compensate for the effects of sleep deprivation.
Sleep deprivation mimics aging?
A report in the medical journal The Lancet, said that cutting back from the standard eight down to four hours of sleep each night produced striking changes in glucose tolerance and endocrine function that mimicked many of the hallmarks of aging. Dr Eve Van Cauter, professor of medicine at the University of Chicago and director of the study, said, "We suspect that chronic sleep loss may not only hasten the onset but could also increase the severity of age-related ailments such as diabetes, hypertension, obesity and memory loss."
Is sleep necessary to consolidate memories?
This is the big question, still being argued by the researchers. . Most of the research favoring sleep’s importance in consolidation has used procedural / skill memory — sequences of actions.
From this research, it does seem that it is the act of sleep itself, not simply the passage of time, that is critical to convert new memories into long-term memory codes.
What sleep does for memory
The findings reinforce the hypothesis that sleep is needed to clear the brain's short-term memory storage and make room for new information.
Sleep helps reduce errors in memory
A study in which college students were shown lists of words and then, 12 hours later, asked to identify which words they had seen or heard earlier, found that those who trained at night and tested the following morning were less prone to falsely recognizing semantically similar words than those who trained in the morning and tested in the evening. It’s suspected that sleep may help strengthen the source of the memory, thus helping protect against false
Memories practiced throughout the day, not just while sleeping
A study investigating the role of sleep in creative problem-solving has found that those who experienced REM sleep between two tests performed significantly better on the later test compared to those who simply had a quiet rest, or those who napped but had no REM sleep. The findings support the idea that REM sleep (when dreams occur) has a role in forming new associations. It’s suggested that the process may be facilitated by changes to neurotransmitter systems (cholinergic and noradrenergic) during REM sleep.
Sleep may be important in regulating emotional responses
The findings suggest that the sleeping brain calculates what is most important about an experience and selects only what is adaptive for consolidation .
Sleep may help clear the brain for new learning sleep effects on the human brain. In a recent study have revealed that during sleep the number of new synapses formed during earlier learning decreased. It’s theorised that this activity during sleep is a way of pruning the less relevant and important synapses (clearing away the junk, as it has been conceptualised). Research showing that more learning resulted in longer sleep. It also supports recent Researcher also found synaptic strength increases during the day, then weakens during sleep. The study also identified three genes essential to the links between learning and increased need for sleep.
Sleep helps you learn complicated tasks & recover forgotten The findings indicate that although people may appear to forget much of their learning over the course of a day, a night’s sleep will restore it; moreover, sleep protected the memory from loss over the course of the next day. The findings confirm the role of sleep in consolidating memory for skills, and extends the research to complicated tasks.
Sleep selectively preserves emotional memories
It’s now generally accepted that sleep plays an important role in consolidating memories, A new study has found that sleep had an effect on emotional aspects of a memory. The findings are consistent with the view that the individual components of emotional memory become 'unbound' during sleep, enabling the brain to selectively preserve only that informationit
Aging impairs the 'replay' of memories during sleep
During sleep, the hippocampus repeatedly "replays" brain activity from recent experiences, in a process believed to be important for memory sleep.
A nap can help you learn
Studies show that short period sleep can help you learn.
Brain connections strengthen during waking hours, weaken during sleep
New research provides support for a much-debated theory that we need sleep to give our synapses time to rest and recover. The human brain is said to expend up to 80% of its energy on synaptic activity, constantly adding and strengthening connections in respoat nse to stimulation. The researchers have theorized that we need an ‘off-line period’, when we are not exposed to the environment, to take synapses down. .Study has revealed that synapses — the all-important points of connection between neurons — are very active when the animal is awake and very quiet during sleep. The researchers feel that these findings support the idea that our brain circuits get progressively stronger during wakefulness and that sleep helps to recalibrate them to a sustainable baseline. .n stronger.
Following on from research showing long-term memory is consolidated during sleep through the replaying of recently encoded experiences, a study has found that the particular order in which they were experienced is also strengthened, probably by a replay of the experiences in "forward" direction. The study involved students being asked to learn triplets of words presented one after the other. Those whose recall of the order of the words was tested after sleep showed better recall, but only when they were asked to reproduce the learned words in forward direction.
Sleep protects against interference
A study involving 48 people (aged 18—30) found that those who learned 20 pairs of words at 9pm and were tested at 9am the following morning, after a night’s sleep, performed better than those who learned them at 9am and were tested at 9pm of the same day. Moreover, for those who were given a second list of word pairs to remember just before testing, where the first word in each pair was the same as on the earlier list, the advantage of sleep was dramatically better. For those who experienced the interference manipulation, those in the sleep group recalled 12% more word pairs than the wake group, but with interference, the recall rate was 44% higher for the sleep group.

Sleeping helps us put facts together
And in yet another sleep study, researchers found evidence that sleep also helps us see the big picture.
Asleep or awake we retain memory
How sleep improves memory
While previous research has been conflicting, it does now seem clear that sleep consolidates learning of motor skills in particular. A new imaging study involving 12 young adults taught a sequence of skilled finger movements has found a dramatic shift in activity pattern when doing the task in those who were allowed to sleep during the 12 hour period before testing. Increased activity was found in the right primary motor cortex, medial prefrontal lobe, hippocampus and left cerebellum — this is assumed to support faster and more accurate motor output. Decreased activity was found in the parietal cortices, the left insular cortex, temporal pole and fronto-polar region — these are assumed to reflect less anxiety and a reduced need for conscious spatial monitoring. It’s suggested that this is one reason why infants need so much sleep — motor skill learning is a high priority at this age. The findings may also have implications for stroke patients and others who have suffered brain injuiries.
Sleep helps insight
A new German study provides evidence for what we all suspected — “sleeping on” a problem can really work. In the study, participants were given a mathematical puzzle to solve; a puzzle which could be solved by trial-by-trial learning, or almost immediately if participants grasped the hidden rule. After training in the trial-by-trial learning, some of the participants were allowed to sleep through the night, while others were prevented from sleeping. When they returned to the problem eight hours later, those that had slept were twice as likely to realize the rule. Another group that trained in the morning, and were then tested later that day, were also slower at finding the rule, suggesting that the slowness was not solely due to fatigue. Sleep did not, however, help participants who had not had the initial training. It is suggested that sleep can act to restructure new memory representations.

Stages of memory clarified in sleep studies
Two new studies add to our understanding of the effects of sleep on memory. Both studies involved young adults and procedural (skill) learning, and found temporary declines in performance in particular contexts (a brief description of these studies is given here). On the basis of these studies, researchers identified three stages of memory processing: the first stage of memory — its stabilization — seems to take around six hours. During this period, the memory appears particularly vulnerable to being “lost”. The second stage of memory processing — consolidation — occurs during sleep. The third and final stage is the recall phase, when the memory is once again ready to be accessed and re-edited. (see my article on consolidation for more explanation of the processes of consolidation and re-consolidation). The surprising aspect to this is the time it appears to take for memories to initially stabilize. The studies also confirm the role of sleep in the consolidation
New motor skills consolidated during sleep
An imaging study that sheds light on the gain in performance observed during the day after learning a new task. Following training in a motor skill, certain brain areas appear to be reactived during REM sleep, resulting in an optimization of the network that subtends the subject's visuo–motor response.

Deep "slow wave" sleep necessary to consolidate memories
Sleep is necessary to consolidate memories. Remembering a new task is more difficult if you don't sleep within 30 hours of learning the task. "Catch-up" sleep on subsequent nights doesn't make up for losing that first night's sleep. Moreover, it appears that the deep "slow wave" sleep that occurs in the first half of the night is the type of sleep necessary to consolidate memories. Other types of memory however, may require "REM" sleep (that occurs while you are dreaming).
What sleep does for memory
A midday nap markedly boosts the brain's learning capacity
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 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 iYs 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).

Helping memory consolidation while you sleep
The role of sleep in consolidating new learning is now well-established, but now a study intriguingly reveals that you can improve that learning by playing sounds associated with the learning while you are asleep.
How sleep consolidates memory
A rat study provides clear evidence that "sharp wave ripples", brainwaves that occur in the hippocampus when it is "off-line", most often during stage four sleep, are responsible for consolidating memory and transferring the learned information from the hippocampus to the neocortex, where long-term memories are stored. The study found that when these waves were eliminated during sleep, the rats were less able to remember a spatial navigation task.

Sleep may help clear the brain for new learning
Sleep helps you learn complicated tasks & recover forgotten skills
Sleep selectively preserves emotional memories
Aging impairs the 'replay' of memories during sleep
During sleep, the hippocampus repeatedly "replays" brain activity from recent experiences, in a process believed to be important for memory consolidation. A new rat study has found reduced replay activity during sleep in old compared to young rats, and rats with the least replay activity performed the worst in tests of spatial memory.
Sleep reinforces the temporal sequence in memory

Sleeping helps us put facts together
And in yet another sleep study, researchers found evidence that sleep also helps us see the big picture. .
Sleep makes memories resistant to interference
Asleep or awake we retain memory
Mentally, sleep may be as active a state as waking state
Why do we sleep? A question we keep asking. Recent research leads us another step in the road. The study has identified a number of genes upregulated specifically during sleep – at least as many as are turned on while we are awake. These "sleep genes" largely fall into four categories: genes involved in synaptic plasticity (supporting the view that sleep aids memory consolidation); genes underlying translation (supporting observations that protein synthesis increases during sleep); genes regulating membrane and vesicle trafficking; and genes for synthesizing cholesterol (which may be crucial for synapse formation and maintenance, which could, in turn, enhance neural plasticity (the brain's ability to change and learn)). The study also found, to the researchers’ surprise, that the cerebellum showed largely the same pattern of gene-expression during sleep as the cortex.
Sleep can act to restructure new memory
Napping reverses information overload
Improving motor skills through sleep

New motor skills consolidated during sleep
Deep "slow wave" sleep necessary to consolidate memories
"Our previous studies demonstrated that a period of sleep could help people improve their performance of 'memory tasks.
these finger movements while an MRI measured the activity of their brain.
"The cerebellum, which functions as one of the brain's motor centers controlling speed and accuracy, was clearly more active when the subjects had had a night of sleep," he explains. At the same time, the MRIs showed reduced activity in the brain's limbic system, the region that controls for emotions, such as stress and anxiety.
"The MRI scans are showing us that brain regions shift dramatically during sleep," says Walker. "When you're asleep, it seems as though you are shifting memory to more efficient storage regions within the brain. Consequently, when you awaken, memory tasks can be performed both . accomplished.
This new research may explain why children and teenagers need more sleep than adults and, in particular, why infants sleep almost round the clock.
"Sleep appears to play a key role in human development," says Walker. "At 12 months of age, infants are in an almost constant state of motor skill learning, coordinating their limbs and digits in a variety of routines. They have an immense amount of new material to consolidate and, consequently, this intensive period of learning may demand a great deal of sleep."
The new findings may also prove to be important to patients who have suffered brain injuries, for example, stroke patients, who have to re-learn language, limb control, etc.
"Perhaps sleep will prove to be another critical factor in a stroke patient's rehabilitation," he notes, adding that in the future he and his colleagues plan to examine sleep disorders and memory disorders to determine if there is a reciprocal relationship between the two.
"If you look at modern society, there has in recent years been a considerable erosion of sleep time.Busy individuals often shortchange their sleep during the week -- purging, if you will -- only to try to catch up by "binging" on sleep on the weekends.
"This is especially troubling considering it is happening not just among adults, but also among teenagers and children," he adds. "Our research is demonstrating that sleep is critical for improving and consolidating procedural skills and that you can't short-change your brain of sleep and still learn effectively."
This study was supported by grants from the National Institutes of Health and the Dana Foundation.
Beth Israel Deaconess Medical Center is a patient care, teaching and research affiliate of Harvard Medical School, and ranks third in National Institutes of Health funding among independent hospitals nationwide. BIDMC is clinically affiliated with the Joslin Diabetes Center and is a research partner of Dana-Farber/Harvard Cancer Center. BIDMC is the official hospital of the Boston Red Sox. For more information, visit
"Let me sleep on it" may just be the right advice for remembering complex information. Getting a good night's rest not only helps retention, but may even help us recall thoughts forgotten during the day, according to researchers from the University of Chicago.
Howard Nusbaum, a professor of psychology at the university, studied the effects of sleep on memory by testing the retention of words. Using a synthesizer, he distorted tapes of recorded speech, making the words difficult to understand. He then played the tapes back to college students, asking them to decipher the speech.
On first exposure, students understood just 21 percent of the words. After an hour of training, they understood 54 percent. "It is something like learning how to understand someone with a foreign accent," explains Nusbaum.
He then ran the same experiment with a second group of students, who were tested first at 9 in the morning and then tested again at 9 that night. This group remembered only remembered 31 percent of the words in the test at the end of the day. Yet after a night of sleep, their scores climbed up again: the following morning, the same students remembered 40 percent of the words.Top of Form

A third group was tested at 9:00 in the evening, then again the next morning. Their retention was also 40 percent.
Apparently, in the process of cleaning up our scattered thoughts, sleep also finds the ones that were about to slip through the cracks. "Sleep might strengthen relevant associations and weaken irrelevant associations, improving access to memories," Nusbaum notes.

The Learning Process and Sleep

Healthy sleep is essential for optimal learning and memory function.
Sleep, learning, and memory are complex phenomena that are not entirely understood. However, animal and human studies suggest that the quantity and quality of sleep have a profound impact on learning and memory. Research suggests that sleep helps learning and memory in two distinct ways. First, a sleep-deprived person cannot focus attention optimally and therefore cannot learn efficiently. Second, sleep itself has a role in the consolidation of memory, which is essential for learning new information.

Although the exact mechanisms are not known, learning and memory are often described in terms of three functions. Acquisition refers to the introduction of new information into the brain. Consolidation represents the processes by which a memory becomes stable. Recall refers to the ability to access the information (whether consciously or unconsciously) after it has been stored.

Each of these steps is necessary for proper memory function. Acquisition and recall occur only during wakefulness, but research suggests that memory consolidation takes place during sleep through the strengthening of the neural connections that form our memories. Although there is no consensus about how sleep makes this process possible, many researchers think that specific characteristics of brainwaves during different stages of sleep are associated with the formation of particular types of memory.

Sleep, Learning, and Memory
Sleep researchers study the role of sleep in learning and memory formation in two ways. The first approach looks at the different stages of sleep (and changes in their duration) in response to learning a variety of new tasks. The second approach examines how sleep deprivation affects learning. Sleep deprivation can be total (no sleep allowed), partial (either early or late sleep is deprived), or selective (specific stages of sleep are deprived).
Sleep Stages and Types of Memory
Different types of memories are formed in new learning situations. Scientists are exploring whether there is a relationship between the consolidation of different types of memories and the various stages of sleep.
The earliest sleep and memory research focused on declarative memory, which is the knowledge of fact-based information, or "what" we know . In one research study, individuals engaged in an intensive language course were observed to have an increase in rapid-eye-movement sleep, or REM sleep. This is a stage of sleep in which dreaming occurs most frequently. Scientists hypothesized that REM sleep played an essential role in the acquisition of learned material. Further studies have suggested that REM sleep seems to be involved in declarative memory processes if the information is complex and emotionally charged, but probably not if the information is simple and emotionally neutral.

Researchers now hypothesize that slow-wave sleep (SWS), which is deep, restorative sleep, also plays a significant role in declarative memory by processing and consolidating newly acquired information. Studies of the connection between sleep and declarative memory have had mixed results, and this is an area of continued research.
Sleep plays a major role in the ability to learn new tasks that require motor coordination and performance.
Research has also focused on sleep and its role in procedural memory—the remembering "how" to do something (for example, riding a bicycle or playing the piano). REM sleep seems to plays a critical role in the consolidation of procedural memory. Other aspects of sleep also play a role: motor learning seems to depend on the amount of lighter stages of sleep, while certain types of visual learning seem to depend on the amount and timing of both deep, slow-wave sleep (SWS) and REM sleep
The Impact of Sleep Deprivation on Learning and Performance lack of adequate sleep has on learning and memory. When we are sleep deprived, our focus, attention, and vigilance drift, making it more difficult to receive information. Without adequate sleep and rest, over-worked neurons can no longer function to coordinate information properly, and we lose our ability to access previously learned information.

In addition, our interpretation of events may be affected. We lose our ability to make sound decisions because we can no longer accurately assess the situation, plan accordingly, and choose the correct behavior. Judgment becomes impaired.

Being chronically tired to the point of fatigue or exhaustion means that we are less likely to perform well. Neurons do not fire optimally, muscles are not rested, and the body’s organ systems are not synchronized. Lapses in focus from sleep deprivation can even result in accidents or injury.

For more information about how sleep deprivation affects performance, see Sleep, Performance, and Public Safety.

Low-quality sleep and sleep deprivation also negatively impact mood, which has consequences for learning. Alterations in mood affect our ability to acquire new information and subsequently to remember that information. Although chronic sleep deprivation affects different individuals in a variety of ways (and the effects are not entirely known), it is clear that a good night’s rest has a strong impact on learning and memory.
Open Questions
Although current research suggests that sleep is essential for proper memory function, there are unanswered questions, as in any area of active scientific inquiry. For example,
Not all researchers are convinced that sleep plays as prominent a role in memory consolidation as others believe. In experiments in which animals completed a course through a complicated maze, the animals' amount of REM sleep increased after performing the task. Some researchers believe that the increase in REM sleep reflects an increased demand on the brain processes that are involved in learning a new task. Other researchers, however, have suggested that any changes in the amount of REM sleep are due to the stress of the task itself, rather than a functional relationship to learning.

Researchers are likewise split with regard to the impact of sleep deprivation on learning and memory. For example, rats often perform much worse on learning tasks after being selectively deprived of REM sleep. This suggests that REM sleep is necessary for the animals’ ability to consolidate the memory of how to perform the task. Some scientists have argued that the observed differences in learning are not actually due to the lack of REM sleep, but may be due to the animals not being as well rested because they were deprived a portion of their sleep.

We still don't know the exact mechanism of the memory process that occurs during sleep, but the results of this research suggest the possibility that it is possible to speed up memory consolidation, and in the future, we may be able to do it artificially," said Prof. Karni.
Long term memory is defined as a permanent memory that doesn't disappear or that disappears after many years. This part of our memory is divided into two types – memories of "what" (for example: what happened yesterday or what one remembers from an article one read yesterday) and memories of "how to" (for example: how to read Hebrew, how to drive, play basketball or play the piano).
Two groups of participants in the study practiced a repeated motor activity which consisted of bringing the thumb and a finger together at a specific sequence. The research examined the "how" aspect of memory in the participants' ability to perform the task quickly and in the correct sequence. One of the groups was allowed to nap for an hour and a half after learning the task while the other group stayed awake.
The group that slept in the afternoon showed a distinct improvement in their task performance by that evening, as opposed to the group that stayed awake, which did not exhibit any improvement. Following an entire night's sleep, both groups exhibited the same skill level. "This part of the research showed that a daytime nap speeds up performance improvement in the brain. After a night's sleep the two groups were at the same level, but the group that slept in the afternoon improved much faster than the group that stayed awake,"
A second experiment showed that another aspect of memory consolidation is accelerated by sleep. It was previously shown that during the 6-8 hours after completing an effective practice session, the neural process of "how" memory consolidation is susceptible to interference, such that if, for example, one learns or performs a second, different task, one's brain will not be able to successfully remember the first trained task.
of their performance, as if there had been no interference at all.
"This part of the study demonstrated, for the first time, that daytime sleep can shorten the time "how to" memory becomes immune to interference and forgetting. Instead of 6-8 hours, the brain consolidated the memory during the 90 minute nap," explains Prof. Karni who added that while this study demonstrates that the process of memory consolidation is accelerated during daytime sleep, it is still not clear which mechanisms sleep accelerates in the process.
The elucidation of these mechanisms, say the researchers, could enable the development of methods to accelerate memory consolidation in adults and to create stable memories in a short time. Until then, if you need to memorize something quickly or if your schedule is filled with different activities which require learning "how" to do things, it is worth finding the time for an afternoon nap.
Source: University of Haifa

There are a number of theories about why we sleep and what happens in our bodies and brains during sleep cycles. However, there is not currently one predominant theory. It is quite possible that there is a kernel of truth in each of these theories; that they will all work together eventually to inform a more complete understanding of human sleep.
Most scientists agree that one of the major purposes of sleep is to restore and heal the body. It has been observed that hormone and immune functions change during specific stages of the sleep cycle. Furthermore, some studies have shown that sleep deprivation can lead to deficiencies in the immune system. Although it is believed by some that important growth can take place during sleep, there have been no studies to show that the lack of sleep can halt or stunt growth.
It has also been hypothesized that sleep offers important restoration to the brain. It is possible that neurons are restored, that brain proteins and certain hormones are produced. Some scientists believe that sleep is particularly important to the brains in young humans.
A completely alternate theory to those described above is the “Preservation and Protection” theory of sleep. This theory asserts that human beings do not require the full 24 hour period within each day to satisfy basic needs such as collecting necessary food and supplies, eating, and reproducing. As not all 24 hours are required, sleep offers a time of rest when humans are not out in the elements, and therefore exposed to threats.
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