Dr. Neubauer is associate director of the Johns Hopkins Sleep Disorders Center and assistant professor in the Department of Psychiatry at the Johns Hopkins University School of Medicine in Baltimore, Maryland. He is also medical director of the Psychiatry Mobile Treatment Program at the Johns Hopkins Bayview Medical Center.

Disclosure: Dr. Neubauer is consultant to sanofi-aventis and Takeda.

Please direct all correspondence to: David N. Neubauer, MD, Johns Hopkins Bayview Medical Center, 4940 Eastern Ave, Box 151, Baltimore, MD 21224.


 

Forgetfulness is a common complaint of people who do not get enough sleep, whether it is due to sleep deprivation from an overly busy schedule or the result of insomnia. People often find that it is more difficult both to learn and remember things when they are unable to achieve sufficient sleep on a regular basis. Although recorded observations about the benefits of sleep on memory date back to ancient Rome, recent research has provided exciting evidence related to different types of memory and the role sleep can perform in learning and memory consolidation. Research findings now suggest that different types of sleep can affect different types of memory. It appears that sleep also can influence emotional aspects of our memory. Some studies1 even suggest that certain types of memory can be enhanced during sleep with instrumentation capable of promoting greater slow wave activity.

While it is well established that previous sleep enhances learning and subsequent sleep helps consolidate memory, many details of the complex sleep-memory relationship still need to be elucidated. Fortunately, the various forms of memory and stages of sleep allow a variety of hypotheses that can be tested and examined with neuroimaging techniques. Memory processes typically are described as either declarative or procedural (nondeclarative). Declarative memories represent fact-based information that is consciously accessible. Declarative memory processes may be divided further into episodic (spatial-temporal context) and semantic (fact information) types. Procedural memory relates to actions, skills, and habits that are not necessarily available for conscious recollection. These all can be differentiated with specific learning tasks. Further, memory can be viewed as having phases, such as encoding or acquisition, consolidation, integration, and recall. To a certain extent, memory types and phases may be affected in different way by different types of sleep, such as rapid eye movement (REM) sleep and slow wave sleep, or by sleep deprivation.2,3

The benefits of adequate sleep prior to learning have been demonstrated in both animal and human studies. Several key brain regions are known to play significant roles in the formation of memories. For example, the hippocampus is important for declarative and spatial learning, while the amygdala is associated with emotional memory. Procedural memory involves striatal and cerebellar functioning. Human neuroimaging investigations of sleep-deprived subjects compared with those having a normal night of sleep show abnormal hippocampal functioning, which may explain their impaired performance with certain learning tasks.1,3

The influence of sleep deprivation on emotional memory was explored in a study by Walker.1 Two groups of subjects, one deprived of sleep for a night and the other allowed a normal night of sleep, were presented with a series of words that the investigators classed as positive, negative, or neutral. The sleep-deprived subjects had worse recall in all three categories with an overall 40% decrease in new memory formation. However, when the word classes were individually considered, the sleep-deprived subjects exhibited surprisingly good retention of the negative items. There was much greater interruption of positive and neutral memory formation in the sleep deprivation group.

Abundant evidence from research studies supports the conclusion that post-learning sleep consolidates both declarative and procedural memory. In some circumstances, sleep can stabilize memory, making it less vulnerable to interference from other learning tasks, and in some studies sleep clearly improves memory performance. Declarative memory typically is assessed with recall tests following the learning of associated word pairs. Procedural memory often is tested with finger sequence tapping tasks or serial reaction time tasks. Nighttime sleep significantly improves memory test performance, but even daytime naps following learning tasks offer subjects benefits compared to individuals tested after the same intervening period without sleep. Memory associated with semantic information, as well as perceptual and sensory motor skills, all have been shown to be enhanced as a result of post-learning sleep. Although further research will be necessary to better define the effects of specific neuroanatomic regions and sleep-related neurophysiologic processes on memory, it is generally accepted that declarative memory consolidation is enhanced with slow wave sleep, while REM sleep specifically benefits procedural and emotional memory. However, it is likely that multiple systems are active during sleep as we process our daily actions and experiences.3

Nishida and colleagues4 examined emotional memory and the amount of REM sleep present during daytime naps. Thirty-one subjects were placed in nap or no-nap groups. Both groups had two study sessions during which they viewed 120 pictures of which one half were neutral and the other half negative in emotional content. The study sessions were 4 hours before and 15 minutes before a 360-picture recognition test. The nap group had a 90-minute sleep opportunity between the two study sessions while the no-nap group subjects remained awake. The results showed that the no-nap group performed about the same in recognizing pictures from the two study sessions. However, the nap group demonstrated much greater recognition of the emotionally negative pictures from the pre-sleep study session compared with the session just prior to the testing. The authors concluded that there was a significant selective sleep facilitation of the emotional memory processing. Furthermore, they found a significant relationship between the emotional memory facilitation and the amount of REM sleep and also with the EEG right-dominant prefrontal theta power.

The relationship of slow wave sleep and memory has been investigated in several clever studies. Backhaus and colleagues5 found that groups of individuals with greater slow wave sleep performed better on declarative memory tests. Aeschbach and colleagues6 markedly reduced slow wave activity but not the total amount of sleep with noise during a post-learning sleep session and demonstrated impaired performance on a visual texture discrimination procedural learning task. Marshall and colleagues7 employed transcranial direct current stimulation to generate slow oscillations characteristic of slow wave sleep in sleeping subjects. The direct current stimulation was associated with increased slow wave activity as well as enhanced performance in declarative memory tests. Rasch and colleagues8 demonstrated enhanced declarative memory performance by associating learning with a cue that was again exposed to the subjects during sleep. The subjects learned a declarative task when exposed to a rose scent or to no odor while awake prior to bedtime. During sleep they were exposed to the same scent cue during different sleep stages or not at all. Only the rose scent re-exposure during slow wave sleep was associated with enhanced memory consolidation.

The vast majority of human research on sleep and memory has been conducted with normal adult subjects. However, some studies have been performed with groups of children or with elderly subjects. Sleep characteristics, such as REM sleep and slow wave sleep, vary dramatically between the very young and very old. Along with the structural brain changes that occur with aging, the sleep alterations may be integrally related with memory functioning. The emerging evidence on the neurophysiology of sleep and memory also raise intriguing questions regarding learning and memory in individuals with sleep abnormalities resulting from medications or other drugs, or in association with medical, psychiatric, and sleep disorders.3

The take-home lessons are fairly simple. Sufficient sleep helps our brains operate optimally while we are awake. One of the important functions of sleep relates to offline memory consolidation, whether it be for information that we consciously learn or more subtle pattern recognition and motor actions. Both declarative and procedural memory processes benefit from the various types of sleep experienced on a typical night. Insufficient sleep impairs memory functioning. Therefore, we should promote adequate sleep on a regular basis and consider the timing of study sessions and sleep opportunities as we attempt to learn new information. If we need to learn something and then recall that information, we should make sure to have adequate sleep before and after the learning. In other words, “cramming” late at night and losing sleep is a poor strategy in preparing for a test. However, reviewing information shortly before bedtime may increase the efficiency of learning. It follows, as well, that disorders which undermine sleep quality should be identified and treated. Improved learning and memory may be among the benefits of the effective treatment of disorders such as sleep apnea and major depressive disorder. PP

References

1.    Walker MP. The role of sleep in cognition and emotion. Ann N Y Acad Sci. 2009;1156:168-197.
2.    Stickgold R, Walker MP. Sleep-dependent memory consolidation and reconsolidation. Sleep Med. 2007;8(4):331-343.
3.    Diekelmann S, Wilhelm I, Born J. The whats and whens of sleep-dependent memory consolidation. Sleep Med Rev. February 28, 2009. [Epub ahead of print].
4.    Nishida M, Pearsall J, Buckner RL, Walker MP. REM sleep, prefrontal theta, and the consolidation of human emotional memory. Cereb Cortex. 2009;19(5):1158-1166.
5.    Backhaus J, Born J, Hoeckesfeld R, Fokuhl S, Hohagen F, Junghanns K. Midlife decline in declarative memory consolidation is correlated with a decline in slow wave sleep. Learn Mem. 2007;14(5):336-341.
6.    Aeschbach D, Cutler AJ, Ronda JM. A role for non-rapid-eye-movement sleep homeostasis in perceptual learning. J Neurosci. 2008;28(11):2766-2772.
7.    Marshall L, Helgadottir H, Molle M, Born J. Boosting slow oscillations during sleep potentiates memory. Nature. 2006;444(7119):610-613.
8.    Rasch B, Buchel C, Gais S, Born J. Odor cues during slow-wave sleep prompt declarative memory consolidation. Science. 2007;315(5817):1426-1429.