The average adult human sleeps between seven and nine hours every night. This equates to approximately one third of an average person’s life. The purpose of sleep for humans has been a highly debated topic. Researchers are curious as to why people sleep and if there is any benefit to sleep. Common theories regarding the purpose of sleep are: energy conservation, body restoration and strengthening memory. The focus of this study is to determine if memory is impacted by sleep. Participants of this study were asked to visit a sleep laboratory once a week for three weeks to determine if interrupting their sleep during specific sleep stages would impair their memory. There are five possible stages of sleep for humans: 1, 2, 3, 4, and Rapid Eye Movement (REM). The major difference in the sleep stages is the amount of brainwave activity that occurs during sleep. To measure the brainwaves in the participants an electroencephalogram (EEG) was used. An EEG is made up of a series of electrodes that when placed on the scalp of a person is able to record the average of their brainwaves. As a person advances from stage 1 through to stage 4 of sleep, the size of the average brainwaves increases and speed decreases. The increase in size occurs when the brain becomes less active because the brainwaves synchronize. This synchrony appears on an EEG as large brainwaves because the brainwaves do not oppose each other. Shorter EEG brainwaves would be caused during high brain activity. This is because the brainwaves would move out of synchrony, which creates a lower average reading on an EEG. The speed decrease occurs because there is less brainwave activity. Stages 3 and 4 are known as Slow-Wave Sleep (SWS) because they have the slowest brainwaves of the sleep stages. REM sleep is a sleep stage that has characteristics of both SWS and the earlier stages of sleep. REM sleep has brainwave activity that is fast, which is more characteristic of stage 1 sleep. In REM the muscles of the body are also highly relaxed, which is characteristic of the SWS stages. REM sleep can also be distinguished from the other stages of sleep because during this stage the eyes of the individual will have periods of rapid movement. To determine what sleep stage a person is in researchers use a Polysomnography (PSG) device which records: the average brainwave size, brainwave speed, and eye movement of the participant.

The researchers used the PSG device to determine what stage of sleep participants were in so they could awaken them during specific stages. During the sleeping period of the study, researchers would awaken the participants when they were in either REM sleep or SWS to see if the specific sleep deprivation had any impact on memory. Researchers started by giving participants memory tasks to complete prior to sleep. The participants would be re-tested on the memory tasks the next morning. The two types of memory tasks that were used were explicit memory tasks and implicit memory tasks. Explicit memory tasks involved the participant recalling information they were shown the night before. An example of recall memory is remembering associated word combinations. Implicit memory tasks involved the participant using procedural memory to complete simple actions they were shown the night before. An example of an implicit memory task would be typing on a keyboard. The researchers thought that depriving SWS would impair explicit memory and depriving REM sleep would impair implicit memory. To determine if there was any brainwave evidence of memory formation after the initial learning phase, the researchers counted the number of “sleep spindles” on the participant’s EEG. A sleep spindle is a burst of 12-14 Hertz waves on an EEG within stage 2 of sleep. The reason for counting sleep spindles was due to past research studies finding a correlation between the number of sleep spindles and the formation of memories. This study was attempting to monitor the learning results and compare them to the number of sleep spindles to determine if they found a correlation.

The results suggested that disruption of specific stages of sleep did have a negative impact on the amount of memory formation participants had the next day. When participants were disrupted during SWS they had more difficulty completing explicit memory tasks such as word association. There was also a negative impact on explicit memory tasks   when participants were disrupted during REM sleep, but the impact was not as great as when SWS was disrupted. The implicit memory of participants was found to not be impacted by the disruption of either REM sleep or SWS. The results also displayed that disruption of SWS, but not REM, negatively impacted participant’s memory of spatial locations. It was also discovered that there was no correlation between the number of sleep spindles and the ability to recall information the next day. This study concludes that both SWS and REM sleep are important for explicit memory in humans. The study also suggests that there are no findings to support a correlation between the number of sleep spindles and either explicit or implicit memory formation.

Reference:

Casey, S. J., Solomons, L. C., Steier, J., Kabra, N., Burnside, A., Pengo, M. F., & … Kopelman, M. D. (2016). Slow wave and REM sleep deprivation effects on explicit and implicit memory during sleep. Neuropsychology, 30(8), 931-945. doi:10.1037/neu0000314