Sleep and memory

Research has shown that sleep is connected to memory. This article discusses research about that topic. I don't have access to the original research study, only the summary article provided in the link. The study setup had participants learn made-up words prior to a night of sleep or the same amount of time awake. The participants were then asked to recall the words after a period of sleep or wakefulness.

The results showed that when compared to wakefulness, sleep helped the participants recall forgotten words. One of the researchers concluded that "sleep almost doubles our chances of remembering previously unrecalled material. The post-sleep boost in memory accessibility may indicate that some memories are sharpened overnight. This supports the notion that, while asleep, we actively rehearse information flagged as important."

CPAP therapy and cognitive performance in elderly patients with severe obstructive sleep apnea

Obstructive sleep apnea (OSA) is more common with age, regardless of weight. OSA in elderly patients can contribute to daytime sleepiness and impair cognitive performance. OSA has even been associated with development of early dementia, called mild cognitive impairment. There is not a lot of data showing the effects of CPAP use on cognitive functioning in elderly patients. But that is just what this study sought to do.

The study population was a subset of a larger study, and participants were at least 65 years old. Participants received extensive neuropsychological testing at the start and end of the study period (6-10 years between the two time points). Participants had a sleep study at the beginning of the study and only those with severe OSA were included. The researchers then divided the participants into those that used CPAP therapy and those that did not. This was a smaller study, with only 126 subjects included, 33 of which used CPAP. Also, note that CPAP use was determined by the patient's self-report. In other words, CPAP use was not determined objectively by checking what the CPAP machine had recorded. So it's possible that CPAP use may have been over-estimated. The researchers also did not separate out CPAP use into the number of hours per night - participants either did or did not report CPAP use. Interestingly, all of the CPAP users reported using CPAP for the minimal 4 hours per night. Finally, participants who used CPAP tended to have more severe OSA, were more overweight, and had more daytime sleepiness. There was no difference in age, gender, or level of education in those that used CPAP versus those that did not.

The results showed that subjects with OSA had a more severe deterioration of cognitive function and memory over time. Also, CPAP therapy helped preserve some cognitive functions like memory, attention, and decision making ability. The authors concluded that long term CPAP use is associated with a protective effect on cognitive performance in elderly patients with severe OSA. They suggest that physicians who treat elderly patients consider screening for and treating OSA to help maintain cognitive performance.

Sleep and learning

Although the exact details are unknown, researchers believe that sleep is helpful in integrating and strengthening new memories. When I was a fellow, it was thought that this occurred mainly in dream (REM) sleep. Here is an article that studied the effect of slow wave sleep (SWS), a deep non-dream state, on memories related to fear. Apparently, studies have shown that memories can be reactivated during SWS by a reminder cue like odors or sounds experienced during SWS.

In this study, researchers used a conditioning procedure in humans to elicit a fear response. Specifically, they repeatedly paired a neutral sound with a mild electrical shock to the wrist of the participants. After enough pairings, the neutral sound would cause a measurable fear response in the participant, even without the electrical shock. In other words, the participant learned that the neutral tone meant the electrical shock was coming. After this conditioned learning, the participants slept for 4 hours because the first 4 hours of sleep are dominated by SWS. The second half of sleep is dominated by REM sleep, and the researchers wanted to isolate the effects of SWS.

One way to eliminate the learned fear response that has become paired with a conditioned stimulus like a neutral tone is to repeatedly expose the subject to the neutral tone but without the electrical shock. Over time, the neutral tone will stop causing the fear response as the subject learns that the electrical shock no longer is associated with the neutral tone. So in the study, the researchers played the neutral tone without the electrical shock during SWS. Results showed that this was able to attenuate the fear response. And this was compared to other subjects that received a different tone than the neutral tone and in those that received no tones at all. Finally, exposure to the tones in SWS did not appear to negatively affect sleep architecture or sleep quality.

So what does all this mean? Well in psychology, when someone has pathological fear or post-traumatic stress disorder, one effective treatment is to repeatedly expose the patient to what they fear, so that their fear response gradually attenuates. It's very challenging therapy for patients, because they have to consciously subject themselves to their fears. But, as this study suggests, maybe patients with pathological fear can be exposed to the feared stimulus during SWS, and not have to experience it consciously in order to reduce the fear response. Here's an analogy...it would be like going from having your appendix taken out while awake to having it taken out under anesthesia.

Sleep and Our Health

Here is an interesting summary article about the importance of sleep and our health. The author discusses how sleep affects memory, the endocrine system, the immune system, and psychiatric disorders. The author uses quotes from a Harvard sleep researcher, Dr. Stickgold, who explains some of the recent findings about sleep and health.

I thought his description of dream importance was a little confusing. In the beginning, he suggests dreams are "much, much less important" than once thought. Then he describes how dreams help us remember and organize recent events. Perhaps he means dreams are less important with regard to analysis of the unconscious?

Slow frequency sounds heard during sleep may improve memory

Slow wave sleep is the deepest level of sleep, occurring mostly in the first half of the night. It's called slow wave sleep because the waveforms 1-3 times per second - contrast this with non-deep sleep, where the waveforms occur 4-7 times per second. This article discusses research showing that sounds produced at the same frequency as slow wave sleep, and listened to while asleep, can enhance word associations. Sounds produced at different frequencies did not improve word associations.

The researchers also think these sounds may improve the overall quality of sleep - more studies are needed though.

Some sleep aids may boost memory by changing sleep

When I perform sleep studies on my patients, one way I have of knowing that a patient is in a certain stage of sleep is by one of their brain wave patterns. One such pattern that is commonly associated with light, non-dream sleep is sleep spindles. Sleep researchers have discovered that certain disorders that can affect memory, like Alzheimer's and schizophrenia, have a reduce amount of sleep spindles. And certain medications can increase the number of spindles. So researchers did a test to see if taking one of these medicine to increase the spindle frequency improved memory. One of the medications was Ambien. The results showed an increase in spindles and an improvement in verbal memory. And the improvement was better than plain sleep. This is exciting, as it may allow us to improve memory in certain people.

Sleep disruption may signal future Alzheimer's Disease

Alzheimer's patients sleep poorly at night and often nap multiple times in the daytime. They can get their days and nights so mixed up, they end up sleeping only in the daytime and are up all night wandering. Sleep problems are a common reason for putting people with Alzheimer's Disease in nursing homes. Here is a study showing that broken up sleep and daytime napping were more likely in people with pre-clinical Alzheimer's Disease - meaning before there is any memory loss. Researchers determined their sleep quality with home based sleep testing and sleep diaries. They determined pre-clinical Alzheimer's Disease with results from a spinal tap. These findings are interesting as a disruption in sleep quality may mean that the person is at risk for developing a cognitive disorder.

Older age people may have memory problems from lack of deep sleep

This article discusses research about our aging brain, sleep quality, and memory. As we age, there is a reduction in volume in a part of our brain behind the middle of our forehead - the medial pre-frontal cortex (PFC). Sleep researchers think this area helps generate the deepest levels of sleep. As we age, the amount of deep sleep is reduced gradually. Many older people have little deep sleep. In this study, the researchers think that the older people who had less deep sleep than younger people, had reduced ability to remember new things. They think this effect is not so much about the volume reduction in the PFC, and more about the loss of deep sleep. Unfortunately, not much can be done to increase the level of deep sleep, whether you are old or young. Some medications have been shown to mildly increase deep sleep levels. Some researchers are apparently working with electrical stimulation of the brain - sounds interesting but I don't know more about it.

Insomnia medications can increase memory and concentration problems

This article discusses how certain medications used for insomnia can increase mild memory and concentration problems in older people. The medications mentioned were benzodiazepines, like valium and anti-histamines, like benadryl but also the "PM" over the counter sleep aids. The study authors acknowledged that some patients may need these medications despite the risk. But at least for insomnia, non-drug treatments have been shown to improve sleep quality. Talk to your doctor or a sleep physician about how you can improve your sleep, rather than resorting to medications.

Sleep and memories

This article discusses research about how our brain processes memory while asleep, even during anesthesia. This is new information and goes against how sleep researchers thought the process worked.

Learning while asleep

This article discusses research showing that some learning can occur during sleep. In this study, researchers exposed subjects to a sound paired with a certain smell that affects breathing during sleep. They did this enough times so that the subjects had the breathing reaction just to the sound alone, even when the smell did not accompany it (think Pavlovian response).

The researchers then tested the subjects when awake and found that they reacted to the sound alone, even when there was no smell along with it. This means that the learning occurred while asleep, as the subjects had no memory of either the sound or smell. Pretty cool, huh?

Sleep may improve exposure therapy in phobias

This article discusses research showing that sleep may improve the effect of exposure therapy. This was done in patients who fear spiders. Results showed that fear was reduced when participants were shown videos of spiders and then allowed to sleep. This makes sense, as sleep is thought to be involved in memory and may allow the participants to remember the video better, and become better desensitized to seeing spiders.

Learned skills may be enhanced with sleep

Here is an article, representative of several I saw about new research about sleep and memories. The article titles imply you can learn things while asleep. But that is not what the research showed. In this study, participants learned to play two different tunes on a keyboard. Researchers played one of the tunes while participants slept and concluded that the participants better recalled the tune played while they slept. The results do NOT mean you can learn things while you sleep. These participants had already learned the tunes - playing the tunes for them only enhanced their recall.

Traumatic Memories, Emotional Reactions, and Sleep

Here is an interesting study printed in the January edition of the Journal of Neuroscience. Researchers showed study subjects neutral and traumatic pictures and assessed their ability to remember them as well as the emotional responses. For one group, they showed the pictures shortly after they woke up and the other group saw them right before going to bed. The results showed that recognition of the pictures was greater when the pictures were shown right before sleep, demonstrating that sleep (particularly dream sleep) is helpful in laying down memories. But, the emotional reactions were less if they were shown the pictures right after getting up and not allowed to sleep. The emotional reaction was the same or even intensified in those that saw the images right before bed - and this was especially true in those that had more dream sleep. This implies that depriving oneself of sleep or at least not sleeping for 12 hours after witnessing a traumatic event could reduce the negative emotional reaction to the event. This could have clinical applications in how psychiatrists treat post-traumatic stress disorder.

Fragmented Sleep and Memory

Researchers published a study at the end of July in the Proceedings of the National Academy of Sciences about the effect of sleep fragmentation and memory in mice. Studies have shown memory problems can occur when sleep is frequently disrupted. However, scientists are not sure if the memory problems are due to shorter total sleep time, poor sleep quality, reduction in a particular sleep stage (e.g. dream or REM sleep), or from the annoyance of being repetitively awaken.

In this study, the researchers used a new technique to isolate the effects of sleep fragmentation from overall sleep quality. The mice's brains were prodded awake every 60 seconds for one night, and this resulted in measurable memory problems. The interesting part is that the frequent awakenings did not reduce REM or deep sleep percentages, the total amount of sleep, or appear to cause the mice any stress.

The researchers suggest that new skills and information are committed to memory during sleep when our brains replay recently learned actions or sequences. The frequent awakenings interrupt that process such that the memories can be lost or compromised before they are stored.

Zolpidem use, Balance and Cognition upon awakening

In the January 13th issue of the Journal of the American Geriatrics Society, there is a study about the effects of Zolpidem on balance and cognition upon awakening. Zolpidem is the active ingredient in Ambien and Ambien CR. This study involved 12 older patients (average age 67 years) and 13 younger adults (average age 22 years). All participants were healthy, physically active, and mentally sharp. Balance was assessed by the tandem walk test, where participants walk on a six-inch beam. Cognition was tested by assessing short term memory and processing speed. The researchers studied balance and cognition right before bedtime and 2 hours after falling asleep after the participants took 5mg of Zolpidem. Researchers also studied balance and cognition after all of the participants went to bed two hours later, to study the effects of partial sleep deprivation without taking Zolpidem.

The results showed that 58% of the older adults and 27% of the younger adults had balance problems after taking 5mg of Zolpidem. Cognition was also impaired in both groups, but the decrease in cognition was more pronounced in younger adults than older adults.

This study underscores the importance of using caution with sleeping pills because of the risk of falls in elderly patients that use these medications. The fact that younger adults had more severe cognitive changes is also important. Five milligrams of Zolpidem is the recommended starting dose for patients older than 65, but 10mg is the more common dosage in younger patients. I wonder if the cognitive changes would be more pronounced at the higher dosage in younger adults.

Your Brain and Sleep Apnea

There is an online article in the American Journal of Respiratory and Critical Care Medicine from October 2010 that links deficits in thinking and brain cells to obstructive sleep apnea (OSA). Apparently this study provides the first evidence that brain cell loss occurs in OSA and that treatment can reverse some of those changes.

Researchers showed that patients with OSA before treatment had impairments in thinking, mood, and had excess sleepiness (as might be expected). Before treatment, the OSA patients had brain cell loss in regions thought responsible for abstract reasoning, executive functioning (planning, organizing, etc), alertness.

The researchers speculate that the areas of the brain affected by OSA get damaged when blood oxygen levels drop too low (called hypoxia). I wonder if there is more to it - in addition to hypoxia, OSA causes surges in adrenaline that occur when OSA patients arouse from sleep to open their airway. This adrenaline surge could spike blood pressure which could do damage to the brain over time. Either way, OSA is bad for the brain.