At the start of the 21st century, scientists had little idea why we sleep, leading J. Allan Hobson at Harvard Medical School to quip that the only known function of sleep was to cure sleepiness. But 20 years later, we know a lot more. It turns out that for every two hours a person is awake and interacting with the world, the brain on average needs to go “offline” for an hour—disconnected from the outside world—to process and contextualize those experiences.
Sleep benefits memory in myriad ways. For simple procedural skills—how to ride a bicycle or distinguish between different coins in one’s pocket—a night of sleep or an afternoon nap following learning leads to a dramatic improvement in performance. Sleep also stabilizes verbal memories, reducing their susceptibility to interference and decay, processes that all too easily lead to forgetting.
But the action of sleep can be more sophisticated than simply strengthening and stabilizing memories. It can lead to the selective retention of emotional memories, or even of emotional components of a scene, while allowing other memories and parts of a scene to fade. It can enhance our ability to extract the gist from a list of words, or the rules governing a complex probabilistic game. It can lead to insights ranging from finding the single word that logically connects three apparently unrelated words, to discovering an unexpected rule that allows for the more efficient solving of mathematical problems. It can facilitate the integration of new information into existing networks of related information. And it has been shown to help infants gain language skills. Disruptions of normal sleep in neurologic and psychiatric disorders can lead to a failure of these processes.
As we describe in our upcoming book When Brains Dream, dreams appear to be part of this ongoing memory processing, and their occurrence and content can predict subsequent memory improvement. While there is a vigorous debate over whether the actual conscious experiencing of dreams while they occur serves a function, we believe that it does, and that it is similar to that proposed for waking consciousness. Antonio Damasio, in this 2000 book The Feeling of What Happens, argues that consciousness provides two critical functions to the human brain: to construct narratives and to feel one’s emotional response to them. Together, they give humans (and presumably other conscious animals) the ability to imagine possibilities, evaluate them, and thereby plan future actions. Our NEXTUP model of dreaming (Network Exploration to Understand Possibilities) proposes that dreaming serves a similar function.
Specifically, we argue that dreaming allows the sleeping brain to enter an altered state of consciousness in which it can construct imagined narratives and respond emotionally to them. While dreaming, the brain identifies associations between recently formed memories (typically from the preceding day) and older, often only weakly related memories, and monitors whether the narrative it constructs from these memories induces an emotional response in the brain. If an emotional feeling is detected, the brain tags the association as potentially valuable, strengthening the link between the two memories and making the association available during subsequent wakefulness.
But dreaming is different from waking consciousness. First, the dreaming brain cannot access and incorporate complete episodic memories (i.e., memories of actual events in our lives), so the associative exploration of dreams is limited to semantic and nondeclarative memories (i.e., memories related to general world knowledge and those acquired and used unconsciously, respectively). In other words, while imagining and planning during wakefulness is normally based on recalled events, narrative construction during dreaming is based on semantic associations of these events, giving dreams their metaphorical quality and allowing for a more expansive investigation of associative links.
Second, the neurochemical modulation of the brain is altered during sleep, and especially during rapid eye movement (REM) sleep, when the release of norepinephrine and serotonin in the brain is shut off while levels of acetylcholine reach their peak in regions such as the hippocampus. These shifts bias memory networks toward the activation of normally weak associations, perhaps explaining the bizarreness of many dreams, especially during REM sleep.
Third, REM sleep is accompanied by a general activation of the limbic system, presumably explaining the enhanced emotionality seen in REM dreams, while also biasing the brain toward creating emotional responses to imagined dream narratives.
Finally, unlike problem solving during wakefulness that relies on imagining and planning, dreaming stops short of offering definitive solutions to our current concerns. Instead, our dreams serve to explore the solution space, helping us to discover new possibilities. It is up to other processes, both in wakefulness and sleep, to draw conclusions and delineate our plans. Dreaming takes what has been and shows us what might be.
Robert Stickgold is a professor at Harvard Medical School and director of the Center for Sleep and Cognition. Antonio Zadra is a professor at the Université de Montréal and a researcher at the Center for Advanced Research in Sleep Medicine. Read an excerpt of When Brains Dream.