Over the past couple of decades, sleep has become recognized as an active state that has profound, diverse effects on memory. The concept that sleep strengthens memory emerged as far back as classical times (for example, the Roman rhetorician Quintilian). Yet, this concept had been marginalized. Instead, a view of sleep as a passive state that simply provides an opportunity for rest held sway. Only in the past quarter of a century have these dominant ideas been challenged by elegant studies demonstrating a link between sleep and memory.

One such study, by Stickgold et al., revealed a connection between sleep and enhanced visual memory. The study tested for the development of improved performance after training and examined the relationship of these ‘offline’ improvements to different brain states. The experiments revealed that it was specifically over a night of sleep that visual memory performance was enhanced (by ~20%). The time spent in rapid eye movement (REM) sleep explained much of this improvement. This observation directly linked a specific offline physiological state to human performance many hours later. The work went further by showing that slow-wave sleep (SWS) during the first quarter of sleep combined with REM sleep during the final quarter explained most of the overnight improvements. This implied a sequential process: changes during SWS followed by changes during REM sleep were together modifying memory. Thus, sleep architecture was shown to have a nuanced and powerful relationship with memory improvements.