Why do memories of vivid dreams disappear soon after waking up?
—Gil Greengross, via e-mail
Ernest Hartmann, professor of psychiatry at Tufts University School of Medicine and director of the Sleep Disorders Center at Newton-Wellesley Hospital, explains: We forget almost all dreams soon after waking up. Our forgetfulness is generally attributed to neurochemical conditions in the brain that occur during REM sleep, a phase of sleep characterized by rapid eye movements and dreaming. But that may not be the whole story.
Perhaps the most compelling explanation is the absence of the hormone norepinephrine in the cerebral cortex, a brain region that plays a key role in memory, thought, language and consciousness. A study published in 2002 in the American Journal of Psychiatry supports the theory that the presence of norepinephrine enhances memory in humans, although its role in learning and recall remains controversial.
A lack of norepinephrine, however, does not completely explain why we forget dreams so easily. Recent research suggests that dreaming lies on a continuum with other forms of mental functioning, which are all characterized by activity in the cerebral cortex. On the one side of this continuum is concentrated, focused thought; dreaming and mind wandering lie on the other, with some overlap among the types. The dreaming/reverie end involves some of the most creative and “far out” material. This type of less consciously directed thinking, however, is not easy to remember. Can you recall where your mind wandered while you were brushing your teeth this morning?
In general, we are very good at forgetting nonessentials. In fact, many of our thoughts, not just those we have while dreaming, are lost. We tend to recall only things that we think about often or that have emotional significance—a problem, a date, a meeting. Mulling over important thoughts activates our dorsolateral prefrontal cortex (DLPFC), a brain region that facilitates memory.
Although most dreams vanish, certain ones tend to remain. These dreams were so beautiful or bizarre, they captured our attention and increased activity in our DLPFC. Thus, the more impressive your dream or thought, the more likely you are to remember it.