When we sleep, we undergo different stages of sleep. The deepest stage of sleep is REM (rapid eye movement) sleep. It has been called so because it can be recognized by rapid, random eye movements. This stage of sleep normally occurs in the early hours of the morning. This particular stage has been linked to dreaming, but it may serve basic functions as well.
It may restore the brain chemistry to a normal balance, stimulate the central nervous system and help form new memories. It may also help emotional adaption, compensate for non-REM sleep activities, mature the cerebral cortex and much more (Smith et al., 2004).
The role of REM sleep in memory consolidation is considered in greater detail here. One way to measure the role of REM sleep in memory consolidation is to let people acquire new skills (i.e., challenge their memory) and then afterwards measure how this skill acquisition affects their REM sleep.
Indeed, a review of seven studies have reported that REM sleep deprivation, after acquisition of procedural learning tasks, resulted in subsequent memory deficits. Another review found increases in time spent in REM sleep after procedural task acquisition (Smith et al., 2004).
“It seems reasonable to assume that the REM sleep state provides an efficient environment for the further processing of recently learned material. During REM sleep, the individual subject apparently engages in specialized posttraining activity, including neuronal replay of recently learned material.” (p. 717).
Studies of rats have shown differences in the magnitude of REM sleep after skill acquisition. The rats that were “fast learners” showed marked increases in number of minutes of REM sleep, whereas the “slow learners” showed small increases in the number of REM sleep, although they did learn equally well. Here comes the interesting part: When the task became very difficult, only “fast learners” were able to master the difficult task.
In other words, the REM sleep increases, as seen in the “fast learners”, were believed to result in better learning and skill acquisition. This finding illustrates how important REM sleep is for memory, and accordingly the learning potential (and possibly intelligence).
Although all rats learned the simple tasks to the same level, only the ones with marked increases in REM sleep were able to learn the most difficult task (Smith et al., 2004).
Smith and colleagues (2004) conducted a study of 24 participants. The authors wanted to examine how REM sleep influences memory consolidation and learning potential. They used a variety of methods to examine REM sleep activity.
For example, they used EEG (electroencephalography) to measure the brain activity, while participants were asleep. Furthermore, they measured the eye movements of both eyes, and at last, they measured the activity of the participants’ chin muscles with the help of EMG (electromyography).
The study shows that both the number and density of REM sleep increases after acquisition of cognitive procedural tasks. The study also shows a similar pattern to the rat study above, in that individuals with the highest IQ scores responded to the tasks with the largest increase in number of REMs. It was suggested that:
“More intelligent individuals would exhibit a larger increase in number of REMs to a modestly difficult task and an even higher number of REMs for a more difficult task.” (p. 717).
In rats, as in humans, time in REM, number of REMs, and REM density have all been reported to increase after a successful task acquisition. REM sleep may therefore provide a biological indicator of learning potential.