What is the relationship between our waking experiences and how they are incorporated into our dreams? Further, how does dreaming play a role in solidifying past events as memories? In the late 19th century, Freud coined the term “day residue” to refer the way in which our experiences on any given day can become incorporated into our dreams that night. Over the years many researchers have confirmed that Freud’s theory actually does reflect how information encoded during the day is replayed and strengthened during the immediate night’s sleep, a critical part of memory consolidation.
Researchers have found that events tend to reappear in our dreams five to seven days after we experience them—an occurrence known as the ‘dream-lag effect’. A recent study found that experiences considered to be personally significant—such as an argument with a loved one—appeared in dreams immediately after their occurrence and then reappeared in the five-to-seven-day window. This study also focused on when in the dream cycle is dream lag most present. While day residue has been recorded in dreams from all parts of the sleep cycle, researchers found the dream-lag effect only in dreams occurring during periods of REM sleep.
These findings help confirm dream lag as a unique function of REM sleep in the integrating of memories. Although researchers agree that all stages of sleep are important for memory consolidation, REM sleep seems to play a specific role in memory evolution, adding new experiences into a growing network of associated memories from the past.