Robert Stickgold

Sleep-dependent memory consolidation.

The concept of ‘sleeping on a problem’ is familiar to most of us. But with myriad stages of sleep, forms of memory and processes of memory encoding and consolidation, sorting out how sleep contributes to memory has been anything but straightforward. Nevertheless, converging evidence, from the molecular to the phenomenological, leaves little doubt that offline memory reprocessing during sleep is an important component of how our memories are formed and ultimately shaped.

Practice with Sleep Makes Perfect: Sleep-Dependent Motor Skill Learning

Improvement in motor skill performance is known to continue for at least 24 hr following training, yet the relative contributions of time spent awake and asleep are unknown. Here we provide evidence that a night of sleep results in a 20% increase in motor speed without loss of accuracy, while an equivalent period of time during wake provides no significant benefit. Furthermore, a significant correlation exists between the improved performance overnight and the amount of stage 2 NREM sleep, particularly late in the night. This finding of sleep-dependent motor skill improvement may have important implications for the efficient learning of all skilled actions in humans.

Dissociable stages of human memory consolidation and reconsolidation.

Historically, the term ‘memory consolidation’ refers to a process whereby a memory becomes increasingly resistant to interference from competing or disrupting factors with the continued passage of time. Recent findings regarding the learning of skilled sensory and motor tasks (‘procedural learning’) have refined this definition, suggesting that consolidation can be more strictly determined by time spent in specific brain states such as wake, sleep or certain stages of sleep. There is also renewed interest in the possibility that recalling or ‘reactivating’ a previously consolidated memory renders it once again fragile and susceptible to interference, therefore requiring periods of reconsolidation. Using a motor skill finger-tapping task, here we provide evidence for at least three different stages of human motor memory processing after initial acquisition. We describe the unique contributions of wake and sleep in the development of different forms of consolidation, and show that waking reactivation can turn a previously consolidated memory back into a labile state requiring subsequent reconsolidation.

Dreaming and the brain: Toward a cognitive neuroscience of conscious states

Sleep researchers in different disciplines disagree about how fully dreaming can be explained in terms of brain physiology. Debate has focused on whether REM sleep dreaming is qualitatively different from nonREM (NREM) sleep and waking. A review of psychophysiological studies shows clear quantitative differences between REM and NREM mentation and between REM and waking mentation. Recent neuroimaging and neurophysiological studies also differentiate REM, NREM, and waking in features with phenomenological implications. Both evidence and theory suggest that there are isomorphisms between the phenomenology and the physiology of dreams. We present a three-dimensional model with specific examples from normally and abnormally changing conscious states.

Sleep-Dependent Learning and Memory Consolidation

While the functions of sleep remain largely unknown, one of the most exciting and contentious hypotheses is that sleep contributes importantly to memory. A large number of studies offer a substantive body of evidence supporting this role of sleep in what is becoming known as sleep-dependent memory processing. This review will provide evidence of sleep-dependent memory consolidation and sleep-dependent brain plasticity and is divided into five sections: (1) an overview of sleep stages, memory categories, and the distinct stages of memory development; (2) a review of the specific relationships between sleep and memory, both in humans and animals; (3) a survey of evidence describing sleep-dependent brain plasticity, including human brain imaging studies as well as animal studies of cellular neurophysiology and molecular biology. We close (4) with a consideration of unanswered questions as well as existing arguments against the role of sleep in learning and memory and (5) a concluding summary.

Visual discrimination learning requires sleep after training.

Performance on a visual discrimination task showed maximal improvement 48–96 hours after initial training, even without intervening practice. When subjects were deprived of sleep for 30 hours after training and then tested after two full nights of recovery sleep, they showed no significant improvement, despite normal levels of alertness. Together with previous findings that subjects show no improvement when retested the same day as training, this demonstrates that sleep within 30 hours of training is absolutely required for improved performance.

Visual Discrimination Task Improvement: A Multi-Step Process Occurring During Sleep

Performance on a visual discrimination task shows longterm improvement after a single training session. When tested within 24 hr of training, improvement, was not observed unless subjects obtained at least 6 hr of postraining sleep prior to retesting, in which case improvement was proportional to the amount of sleep in excess of 6 hr. For subjects averaging 8 hr of sleep, overnight improvement was proportional to the amount of slow wave sleep (SWS) in the first quarter of the night, as well as the amount of rapid eye movement sleep (REM) in the last quarter. REM during the intervening 4 hr did not appear to contribute to improvement. A two-step process, modeling throughput as the product of the amount of early SWS and late REM, accounts for 80 percent of intersubject variance. These results suggest that, in the case of this visual discrimination task, both SWS and REM are required to consolidate experience-dependent neuronal changes into a form that supports improved task performance.

Sleep-dependent learning: a nap is as good as a night

The learning of perceptual skills has been shown in some cases to depend on the plasticity of the visual cortex and to require post-training nocturnal sleep. We now report that sleep-dependent learning of a texture discrimination task can be accomplished in humans by brief (60– 90 min) naps containing both slow-wave sleep (SWS) and rapid eye movement (REM) sleep. This nap-dependent learning closely resembled that previously reported for an 8-h night of sleep in terms of magnitude, sleep-stage dependency and retinotopic specificity, and it was additive to subsequent sleep-dependent improvement, such that performance over 24 h showed as much learning as is normally seen after twice that length of time. Thus, from the perspective of behavioral improvement, a nap is as good as a night of sleep for learning on this perceptual task.

Dreaming and Episodic Memory: A Functional Dissociation?

The activity that takes place in memory systems during sleep is likely to be related to the role of sleep in memory consolidation and learning, as well as to the generation of dream hallucinations. This study addressed the often-stated hypothesis that replay of whole episodic memories contributes to the multimodal hallucinations of sleep. Over a period of 14 days, 29 subjects kept a log of daytime activities, events, and concerns, wrote down any recalled dreams, and scored the dreams for incorporation of any waking experiences. While 65 of a total of 299 sleep mentation reports were judged to reflect aspects of recent waking life experiences, the episodic replay of waking events was found in no more than 12 of the dream reports. This finding has implications for understanding the unique memory processing that takes place during the night and is consistent with evidence that sleep has no role in episodic memory consolidation.

Sleep Preferentially Enhances Memory for Emotional Components of Scenes

Central aspects of emotional experiences are often well remembered at the expense of background details. Previous studies of such memory trade-offs have focused on memory after brief delays, but little is known about how these components of emotional memories change over time. We investigated the evolution of memory for negative scenes across 30 min, 12 daytime hours spent awake, and 12 nighttime hours including sleep. After 30 min, negative objects were well remembered at the expense of information about their backgrounds. Time spent awake led to forgetting of the entire negative scene, with memories of objects and their backgrounds decaying at similar rates. Sleep, in contrast, led to a preservation of memories of negative objects, but not their backgrounds, a result suggesting that the two components undergo differential processing during sleep. Memory for a negative scene develops differentially across time delays containing sleep and wake, with sleep selectively consolidating those aspects of memory that are of greatest value to the organism.