First, no one is sure why we, or any other animal, dream. During the past century, however, scientists have made significant progress in understanding the factors that influence sleep and dreaming. Evidently, our nights are influenced by what we’re doing during the day. We spend our days gathering information—some of it we would like to keep, but most of it we need to discard. If we do not clear out our mental storage space regularly we risk saturating our brain with too much useless trivia. To fully process this information, we need to shut our brain down—disconnect it entirely from the outside world. The challenge is that we only have one brain to work with. How do mammals solve this problem?

About 2 million years ago (that’s the current guess) brains became sufficiently complex that they were able to solve the problem of memory processing by dividing the night into two types of sleep: rapid eye-movement (REM) sleep and slow wave or non-REM sleep. [See Wenk, 2017, for a more detailed discussion] We can, and do, dream in either phase of sleep but the characteristics of our dreams differ greatly. It appears that the major difference in terms of the content of REM dreams versus non-REM dreams is that REM dreams involve more emotional, visual, and movement content, are longer, show more continuity and coherence, and are more vivid than non-REM dreams. During non-REM........

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