What Determines the Subjects of Our Dreams

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We dream about the events, both real and modified, of our waking life. Even when we dream about the future, we utilize the events, places, and people of the past. About 70% of dream content is drawn from the events of the previous day. (These are called day residue dreams.) Nevertheless, previous events are rarely replayed in their entirety. More often, fragments of these recent events are combined into novel scenarios that consist of a series of loosely connected remote and recent events. Why?

Consolidation of Memories

The fact that dreams focus on events of the very recent past supports a popular hypothesis that dreams are important for the consolidation of memories in the brain for long-term storage. Essentially, dreams are what we experience when the brain is involved in the reactivation and organization of memory traces into a pattern of neuronal connections that, in the future, will interact with each other in a new manner.

Objects and people in dreams are familiar. For example, if you have never met me, then you have never seen me in a dream. Also, no one on this planet has ever seen a true alien from another planet in their dreams. The events of our lives form the components of our dreams; we can mix them up to dream new objects or experiences, but the parts are always familiar. All humans in our dreams are vertically symmetrical bipeds. Also, the content of our dreams often reflects our waking conceptions. If you're a Republican during the day, you're not likely to see yourself as a Democrat in your dreams.

In support of this theory, studies of both rapid eye movement (REM) sleep and non-REM (NREM) sleep have reported that dreaming about recently experienced events or recently learned facts is associated with improved learning and performance. In addition, this process during dreaming, particularly during REM sleep, is thought to create novel associations between recent life events and well-established memories. In addition, dreams that include past events may allow the brain to simulate potential future scenarios and then rehearse potential responses. Dreaming contributes to our survival.

New Study of Memory Types and Dream Timing

A recent study addressed these theories by investigating whether different types of memories are incorporated into dreams at different times of the night or during different sleep stages. Some insights into this process already are known. For example, the most recent memories tend to appear in dreams that occur during the earliest phases of sleep. In contrast, older memories tend to be incorporated into dreams at the end of the night. In the current study, the type of memory involved in different stages of dreaming was further divided into episodic memories that pertained to events that were personally experienced in a certain spatial and temporal context, and semantic memories that reflect general knowledge.

Sleeping is divided into two phases, REM sleep and NREM sleep. NREM sleep is further divided into N1 (light sleep), N2 (deeper sleep), and N3 (deep sleep). Dreams can occur during all phases of sleep; however, dreaming occurs predominantly during REM. Dreams in the N-phases “feel” different from dreams during REM. Day residue events are more frequently incorporated into N2 dreams than during REM. In contrast, older memories of events that occurred up to a year ago are frequently incorporated into REM dreams as compared to N2 dreams. (For more on sleep basics, see my book The Brain: What Everyone Needs to Know.)

Dreams during light sleep (N1) and REM sleep contained both recent and semantic memories, as compared to deeper stages of sleep. Dreaming about both recent and semantic memories simultaneously may allow the brain to assimilate recent experiences into broad autobiographical memories. The brain also uses these recent experiences to provide detailed, sensory-rich, and complex dream worlds, particularly during the long-lasting dreaming that characterizes REM sleep.

The study also discovered that the source of the memories that appear in dreams becomes more remote as the night progresses. Thus, early in the night, the majority of topics in dreams involve recently acquired memories; later in the night, the majority of topics in dreams include more remote memories. This trend happened independent of the sleep stage when the dreams occurred. Why?

To understand these results, it is necessary to consider why the brain sleeps and dreams. After all, being unconscious for many hours every day places one’s survival at risk. One proposed function of sleep and dreaming is to organize and integrate new knowledge with existing knowledge. Unfortunately, it is still not clear how or when this process occurs during sleep. Because relatively more remote memories appear in dreams later in the night, and because we spend more time in REM sleep during the early morning hours, this integration function probably happens at the end of the sleep cycle.

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The fact that recent memories, the potentially meaningless events of the previous day, become less apparent as the night progresses may indicate that they are being erased during the NREM sleep that predominates during the first half of the night. Scientists hypothesize that the purpose of NREM sleep may be the elimination of weak recent memories that are deemed unimportant and to preserve the important ones.

In contrast, some memories reappear multiple times throughout the same night and across multiple nights. These memories, which often have a higher emotional content, may be deemed more relevant and are being preserved.

If the results of this study are correct, they suggest that as the brain progresses through multiple NREM–REM cycles each night, new important memories are being incorporated into autobiographical memory networks that form the basis of our waking experience about our present and past life.

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