New research offers insight into how long-term memories are solidified over time as we sleep.
We tend to think—correctly—that our experiences on any given day can become incorporated into our dreams that night. In the late 19th century, Freud coined the term "day residue" to refer to this phenomenon, and as generations of researchers have affirmed, his words reflect how information encoded during the day is replayed and strengthened during the immediate night's sleep, a critical part of memory consolidation.
Michelle Carr, Ph.D, is a researcher at Swansea University Sleep Laboratory. She holds a PhD in Biomedical Science from the University of Montreal, where she conducted research at the Dream and Nightmare Laboratory. Prior to that, she received dual degrees, a BS in Brain and Cognitive Science and a BA in Psychology, from the University of Rochester in 2010. Her work focuses on the role of REM sleep and dreams in emotional memory, along with comparative studies of nightmare disorder and PTSD. Other research interests include sleep paralysis, lucid dreaming, consciousness studies, and the use of dreamwork in psychology.
Editor: Saad Shaheed
Yet there's another temporal pattern in the way our waking life figures into our dreams. 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. Two studies published last year help shed light on the unique role of dream lag in the basic functioning of the sleeping brain and the way past events are solidified as memories.
In one study, published in Neurobiology of Learning and Memory, 44 participants were asked to keep a 10-day log of their waking experiences before undergoing a night of sleep, either at home or in a laboratory, and reporting their dreams. Their waking experiences were categorized as three types: personally significant events, daily activities, and major concerns. Experiences considered to be personally significant—such as an argument—popped up in dreams immediately after their occurrence and then reappeared in the five- to seven-day window.
"Important events often go into dreams, and we assume these events are being consolidated during REM sleep," says Mark Blagrove, a sleep researcher at Swansea University in the U.K. The findings suggest that memory consolidation in sleep is a kind of triage whereby only certain memories are selected and processed for permanent storage.
The study also focused on when in the sleep cycle dream lag is most present. While day residue has been recorded in dreams from all parts of the sleep cycle, Blagrove found the dream-lag effect only in dreams occurring during periods of REM sleep. The subjects who slept at home were monitored by Nightcaps, medical bands with forehead sensors that assess sleep stages based on both head and eye movements. When subjects had experienced 10 minutes of REM sleep, the band signaled an awakening noise and subjects were prompted to report their dreams. Subjects who slept in the laboratory were awakened from both slow-wave sleep and REM sleep by technicians. Only the at-home REM dream reports showed the dream lag and only for personally significant events.
The findings help confirm dream lag as a unique function of REM sleep in integrating memories. Researchers largely agree that all stages of sleep are important for the offline consolidation of memory, but REM sleep seems to play a specific role in memory evolution, adding new experiences into a growing network of associated memories from the past. And only emotional or personally important memories are targeted by REM sleep.
Other recent work on dream lag suggests a time course of memory evolution based on the content of the dreams themselves. Most people recognize that their dreams seem to merge childhood memories with more recent events. In showing how memories from one or two days ago are connected to memories from five to seven days ago in dreams, researchers are beginning to demonstrate how the pattern spirals outward to reach more remote memories, all of which are gradually absorbed to construct our personal narrative.
In a study published last year in the International Journal of Dream Research, Elizaveta Solomonova, a doctoral student at the Dream and Nightmare Laboratory at the Center for Advanced Research in Sleep Medicine in Montreal, approached the actual experience of spending a night in a sleep laboratory as the kind of personally significant event likely to be processed in dreams. "Sleeping in a lab is an unusual experience," Solomonova explains. "There's vulnerability because you have to sleep and put trust in the experimenters." In her study, the laboratory was incorporated into dreams both immediately and after a delay. The delayed incorporations, however, were less direct.
The morning after sleeping in the laboratory, one participant reported the following: "I wake up and get out of the laboratory bedroom. [It] is exactly the same as I saw it yesterday…somebody is taking the electrodes off my head."
Seven days after sleeping in the lab, another participant's dream referenced the same experience, now layered with other memories: "I am being admitted to a hospital because I am unable to remember my dreams," the participant reported. "A team of doctors stands over the gurney telling me that I must be hospitalized." Both participants' dreams seem to have been influenced by their laboratory experience, but the first was a simple replay of the experience, whereas the second merged the lab with past memories of hospitals and doctors to yield an original, associative story.
In this way, Solomonova posits, individual memory traces become connected over time with more and more discrete memories. If you took a sunny walk in a park yesterday, last night's dream may have referenced the walk in relatively clear detail. But that memory will eventually be related to all sorts of other experiences: picnics in the park last summer, the park near your childhood home, or even nonspecific experiences like the pleasant solitude of a long walk or the warmth of sunlight in summer. "We can better understand how the sleeping brain puts together memories that are temporally distant," says Solomonova. "It uses some elements that are recent but finds links with memories from the past."
The findings about dream lag imply that sleep-based memory consolidation is a complex and gradual process, as recent events are associated with increasingly diverse and distant memories. And it's specifically those personally significant events that are targeted by the process. In this way, our memory and even our sense of self evolves and updates over time as new experiences are incorporated. Perhaps most important, understandingthe dream-lag effect adds to our knowledge of how sleep is involved with memory. "What might be happening to memory," Blagrove says, "is showing itself in the dream."