Why Do We Have Different Kinds of Dreams at Different Stages of the Night?
Dreams aren’t all the same. Everyone is aware of the difference between good and bad dreams, but we don’t tend to notice that some dreams are more logical and structured while others are more bizarre. Some dreams are so highly realistic that it is difficult to convince ourselves they aren’t real, while others are fuzzy and indistinct. Some dreams are fragmented, jumping rapidly from one topic to another, while others move forward in a more coherent story. Recent analyses have suggested that these differences are far from random; instead they may be driven by the physiology of various brain states and the extent to which structures like the hippocampus and neocortex are in communication during different sleep stages.
Dreams occur in all stages of sleep, but they seem to become increasingly fragmented as the night progresses. In general, they appear to be constructed out of a mishmash of prior experience. As mentioned above, dreams contain disconnected memory fragments: places we’ve been, faces we’ve seen, situations that are partly familiar. These fragments can either be pasted together in a semi-random mess or organized in a structured and realistic way. The dreams that occur in non-REM sleep tend to be shorter but more cohesive than REM dreams, and often they relate to things that just happened the day before. REM dreams that occur early in the night often also reflect recent waking experiences, but they are more fragmented than their non-REM counterparts. Conversely, REM dreams that occur late in the night are typically much more bizarre and disjointed.
Simply thinking about where these memory fragments are coming from and how they are connected together may provide an explanation for the difference between early and late-night dreams. The various elements of an episode are thought to be stored in the neocortex, but they are not necessarily linked together to form a complete representation. For example, if your memory of having dinner last night involves memories about a specific place, specific sounds, specific actions, and maybe even memories about other people who were there, each of these bits of information is represented by a different area of the neocortex. Even though they combine together to make up a complete memory, these various neocortical areas may not be directly interlinked. Instead, the hippocampus keeps track of such connections and forms the appropriate linkages, at least while the memory is relatively fresh. However, communication between the neocortex and hippocampus is disrupted during sleep, so this process is also disrupted. During REM sleep, both the hippocampus and those parts of the neocortex which are involved in a current dream are strongly active—but they don’t appear to be in communication. Instead, responses in the neocortex occur independently, without hippocampal input, so they must relate to memory fragments rather than linked multisensory representations. Essentially, when memories which have been stored in the neocortex are accessed or activated during REM, they remain fragmentary instead of drawing in other aspects of the same memory to form a complete episodic replay. These fragments aren’t linked together in the way they might be if you thought of the same place while you were awake (or indeed in non-REM sleep). For instance, cortical representations of both someone who was present for your dinner last night and of the place where it was held may be triggered, but these will not necessarily be linked together, and they may not be linked to the idea of dinner or eating at all. Instead, seemingly unrelated characters and events may be activated in conjunction with the memory of this place. One possible driver for this is the stress hormone cortisol, which increases steadily across the night. High cortisol concentrations can block communication between the hippocampus and neocortex, and since concentrations are much higher early in the morning, this could provide a physiological reason for the disjointed properties of late-night (early morning) dreams.
Irrespective of how it happens, it is clear that dreams not only replay memory fragments but also create brand-new, highly creative mixtures of memories and knowledge. This process has led to the creation of many works of literature, art, and science, such as Mary Shelley’s Frankenstein, the molecular formula of benzene, and the invention of the light bulb. An especially good demonstration of this somnolent creativity comes from a study of 35 professional musicians who not only heard more music in their dreams than your normal man-on-the-street but also reported that much of this (28 percent) was music they had never heard in waking life. They had created new music in their dreams!
Although we don’t quite understand how dreams achieve this type of innovative recombination of material, it seems clear that the sleeping brain is somehow freed of constraints and can thus create whole sequences of free associations. This is not only useful for creativity, it is also thought to facilitate insight and problem solving. It may even be critical for the integration of newly acquired memories with more remote ones (see chapter 8). In fact, this facilitated lateral thinking could, in itself, be the true purpose of dreams. It is certainly valuable enough to have evolved through natural selection.