According to sleep memory theory, dreaming sleep is essential for consolidation of memory.

Daydreaming, when we're awake and quiet may serve the same purpose.

My grandmother, a no-nonsense woman from New England, would sometimes describe a person as “being in a brown study.” It’s an English idiom, not often used today, describing someone in serious (the “study” part of the phrase), somewhat melancholic (“brown”) thought. She was saying that the individual in question was wasting time daydreaming when they should be doing something productive. It turns out that my grandmother may have been wrong—that daydreaming might be very productive after all.

The reason for beneficial daydreams lies with another idiom that you are likely more familiar with, and that is the advice to “sleep on” an idea before deciding what to do. This saying stems from the very old idea (traced to the 1500s) that sleep will help us make better decisions (The Idioms, n.d.). The benefits of sleep for memory have a long history in science. However, as our understanding of the complexity of sleep grew, so did our questions about how sleep might help us solve a problem. Tracing the study of sleep and memory, Stickgold (2005) says that the turning point was a “seminal paper by Karni, Sagi and colleagues” published in 1994. This study found that memory creation was dependent on rapid eye movement (REM or dreaming) sleep. A great deal of research has backed up this idea.

One issue facing researchers is that sleep is complicated, as is memory. Just as there is more than one kind of sleep (dreaming or REM sleep and non-dreaming slow-wave sleep) that involves different systems in the brain, there are also several different kinds of memory, also using different but overlapping parts of the brain. There are two broad categories of memory: Declarative (essentially memories we can talk about using language) and non-declarative (memories that are difficult to apply language to). Remembering facts like the name of your home state’s capital is called semantic declarative memory. Memory for a specific event in your life, along with the context for that event, is called episodic declarative memory. And memory for things like how to ride a bicycle is a form of non-declarative memory (there are other forms as well) called procedural memory.

Memories undergo a process of “consolidation” in the brain, meaning they are made stable, resistant to interference, and long-lasting. Memory consolidation, regardless of the type of memory, happens over time, and traditionally, according to models of the role of sleep in memory formation, REM sleep is particularly important in this process. The current thinking is that memories are consolidated in a structure called the hippocampus using a process of reactivation of the pattern of neuron firing that happened during the initial experience of the event. For example, consider what is called “place learning” in the rat.

When a rat learns to navigate a maze, cells called “place cells” in the hippocampus fire in a pattern that replicates the route needed to reach the goal (reward). Place cells respond at specific locations within the local environment. That pattern of place cell responses is stored at first temporarily in the hippocampus. When the rat is asleep, this ensemble of place cell responses is reactivated, making the connections between cells stronger. Eventually, through repeated reactivations of the pattern, the now very strong memory of the route through the maze is sent to the cortex for long-term storage.

Originally, researchers thought that this reactivation of patterns of cell responses was only happening during sleep. Recent research has suggested that reactivation also happens when we’re awake and in quiet rest, when we’re daydreaming. FMRI studies have shown that this happens in rats after training in a maze, as well as in humans (Tambini and Davachi, 2019), in both the hippocampus and various regions of the cortex thought to be involved in long-term memory storage.

The tasks humans were asked to do were a bit more complex than the relatively simple mazes used to test rodents. For example, Schuck and Niv (2019) examined reactivation responses in the human hippocampus using FMRI. Their participants viewed compound images of faces (young and old) and houses (also young and old) and were asked to judge the age of either the face or the house. They were able to demonstrate that the patterns of responses from the hippocampus during a post-task resting state reflected a replay of the activity seen during this abstract, non-spatial, decision-making task.

Taking advantage of the stronger learning that takes place in a fearful situation, de Voogd, Fernández, and Hermans (2016) asked their participants to look at pictures of either animals or plants. Baseline recordings were made with participants viewing all of the images. Fear training began with 50% of the images in one category (for example, the plant category) being paired with mild shock (the CS+ condition). Half the participants experienced the CS+ with plants, the other half with animal images. The next day, four images from each category were presented, without the shock being present, and recordings were made. They found “increased spontaneous reactivation of category-specific patterns, during post-learning awake rest for a conceptual category that was coupled to an emotionally arousing event”. Pairing shock with plants created a memory of plants as fearful stimuli.

Investigation into the processes used in consolidation is ongoing. The evidence is mounting that daydreams are not just a waste of time. We may well be actively working to remember what happened to us today when we are daydreaming.

Karni, D.T. et al. (1994). Dependence on REM sleep of overnight improvement of a perceptual skill. Science, 265, (5172), 679-682.

Schuck, N.W., and Niv, Y. (2019). Sequential replay of non-spatial task states in the human hippocampus. Science, 364 (6447).

Stickgold, R. (2005). Sleep-dependent memory consolidation. Nature, 437(27), 1272–1287

Tambini, A., and Davachi, L. (2019). Awake reactivation of prior experiences consolidates memories and biases cognition, Trends in Cognitive Science, 23(10), 876-890.

The idioms (n.d.) Sleep On. In TheIdioms.com dictionary. Retrieved April 24, 2026, from theidioms.com.

Voogd, L.D. de, Fernández, G.S.E., and Hermans, E.J. (2016). Awake reactivation of emotional memory traces through hippocampal-neocortical interactions. NeuroImage, 134, 563-572.

There was a problem adding your email address. Please try again.

By submitting your information you agree to the Psychology Today Terms & Conditions and Privacy Policy

© Psychology Today