According to sleep memory theory, dreaming sleep is essential for consolidation of memory.

Daydreaming, when we're awake and quiet may serve the same purpose.

My grandmother, a no-nonsense woman from New England, would sometimes describe a person as “being in a brown study.” It’s an English idiom, not often used today, describing someone in serious (the “study” part of the phrase), somewhat melancholic (“brown”) thought. She was saying that the individual in question was wasting time daydreaming when they should be doing something productive. It turns out that my grandmother may have been wrong—that daydreaming might be very productive after all.

The reason for beneficial daydreams lies with another idiom that you are likely more familiar with, and that is the advice to “sleep on” an idea before deciding what to do. This saying stems from the very old idea (traced to the 1500s) that sleep will help us make better decisions (The Idioms, n.d.). The benefits of sleep for memory have a long history in science. However, as our understanding of the complexity of sleep grew, so did our questions about how sleep might help us solve a problem. Tracing the study of sleep and memory, Stickgold (2005) says that the turning point was a “seminal paper by Karni, Sagi and colleagues” published in 1994. This study found that memory creation was dependent on rapid eye movement (REM or dreaming) sleep. A great deal of research has backed up this idea.

One issue facing researchers is that sleep is complicated, as is memory. Just as there is more than one kind of sleep (dreaming or REM sleep and non-dreaming slow-wave sleep) that involves different systems in........

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