page_title: Sleep and memory
section_title: Sleep cycles
hierarchical_section_title: Sleep and memory / Sleep cycles
caption_attribution_description: English: Graph showing the passage through the four principle phases of sleep over the course of a night. Portions marked in red indicate REM sleep.
context_page_description: The relationship between sleep and memory has been studied since at least the early 19th century. Memory, the cognitive process of storing and retrieving past experiences, learning and recognition, is a product of brain plasticity, the structural changes within synapses that create associations between stimuli. Stimuli are encoded within milliseconds; however, the long-term maintenance of memories can take additional minutes, days, or even years to fully consolidate and become a stable memory that is accessible. Therefore, the formation of a specific memory occurs rapidly, but the evolution of a memory is often an ongoing process.
  Memory processes have been shown to be stabilized and enhanced and memories better consolidated by nocturnal sleep and daytime naps. Certain sleep stages have been demonstrated as improving an individual's memory, though this is task-specific. Generally, declarative memories are believed to be enhanced by slow-wave sleep, while non-declarative memories are enhanced by rapid eye movement sleep, although there are some inconsistencies among experimental results.
context_section_description: Sleep progresses in a cycle which consists of five stages. Four of these stages are collectively referred to as non-rapid eye movement (NREM) sleep whereas the last cycle is a rapid eye movement period. A cycle takes approximately 90–110 minutes to complete. Wakefulness is found through an electroencephalogram (EEG) which is measured and characterized by beta waves, the highest in frequency but lowest in amplitude, and tend to move inconsistently due to the vast amount of stimuli a person encounters while awake. 
  Pre-sleep is the period of decreased perceptual awareness where brain activity is characterized by alpha waves which are more rhythmic, higher in amplitude and lower in frequency compared to beta waves.
  Stage one is characterized by light sleep and lasts roughly 10 minutes. Brain waves gradually transition to theta waves.
  Stage two also contains theta waves; however, random short bursts of increased frequency called sleep spindles are a defining characteristic of this stage.
  Stage three and four are very similar and together are considered to be "deep sleep". In  these stages brain activity transitions to delta waves which are the lowest in frequency and highest in amplitude. These two stages combined are also called slow wave sleep (SWS).
  Stage five, REM sleep, is one of the most interesting stages as brain wave patterns are similar to those seen in relaxed wakefulness. This is referred to as "active sleep" and is the period when most dreaming occurs. REM sleep is also thought to play a role in the cognitive development of infants and children as they spend much more of their sleep in REM periods opposed to adults.
  During the first half of the night, the largest portion of sleep is spent as SWS, but as the night progresses SWS stages decrease in length while REM stages increase.
caption: Sleep cycles, with REM indicated in red
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