Journaling reduces intrusive thoughts and improves working memory

James Pennebaker's research at the University of Texas in the late 1980s found that writing about traumatic experiences for just 15–20 minutes a day over four days led to improved immune function, fewer doctor visits, and better academic performance. Later research found the mechanism is cognitive: putting experiences into words forces the brain to organise them into a narrative, reducing the frequency with which intrusive memories interrupt working memory. The effect is strongest when writing includes both facts and feelings.

Source: Pennebaker & Beall, Journal of Abnormal Psychology (1986)