This is the second post in my series on sleep and memory. Check out the first post in the series for an introduction to the four stages of sleep and early research on the interaction of sleep and memory.
Rapid Eye Movement (REM) or dreaming sleep is not exactly restful. Dreaming uses up energy, and our bodies use more oxygen while in REM sleep than when we are awake! Since the purpose of REM sleep can’t be to replenish energy, something else must be going on in the brain. This “something else” is suspected to be the organization and preservation of memories.
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During childhood, 50% of the time spent asleep is devoted to REM sleep. Childhood is also a period of intense learning. As kids, we learn how to walk, how to read, how to make friends, and many other valuable skills we’ll rely on our entire lives. The high amount of REM sleep in childhood is likely crucial for remembering what we learn as kids
In contrast, REM sleep deprivation makes learning difficult. REM sleep deprivation during childhood development is especially detrimental and can cause problems with the visual system and behavioral differences that remain in adulthood. 
REM sleep is also linked to efficient learning since the more REM sleep a person gets after learning the more productively they can learn. To foster effective learning, the length and quality of REM sleep increases after learning in children and younger adults. 
Getting sufficient REM sleep improves task performance and helps people retain memories in challenging circumstances. Here’s an example: in a study by Jeffery Ellenbogen and his colleagues, study subjects were asked to memorize two similar lists of words. Subjects who got normal amounts of REM sleep didn’t confuse the two lists as much as subjects who were deprived of REM sleep. 
The benefits of REM sleep for retaining memories are less prevalent in older adults. The ability to learn and remember things decreases with age as does the duration and quality of REM sleep. Older adults do not show the same increase in REM sleep after learning that younger adults and children do. This is likely linked to the fact that older adults tend to be less efficient learners than younger people.
Most of the evidence of the role of REM sleep in memory retention I’ve discussed comes from sleep disruption studies. Where participants are first taught a new task, told a story, or asked to memorize a list of words. After this, the study subjects’ brain waves are monitored during their sleep. The control group are allowed to have a normal night’s sleep, while the experimental group are woken up as soon as they enter REM sleep, thus depriving them of only REM sleep. Then, the next day the subject’s memory of what they learned is tested to see how REM sleep affected their memory.
The results from these studies seem conclusive—REM sleep is crucial for memory retention! But, there is more than one type of memory. Remembering how to ride a bike requires different brain activity than remembering what you ate for lunch on Tuesday. Does REM sleep help us remember all types of memories or just a few types? Do other sleep phases play a role in memory as well? Find out in my next post.
 Li, Ma, Yang, and W. B. Gan. “REM sleep selectively prunes and maintains new synapses in development and learning.” Nature Neuroscience, vol. 20, Jan. 2017, pp. 427-437. doi: 10.1038/nn.4479.
 Hornung, Regen, Danker-Hopfe, Schredi, and I. Heuser. “The Relationship Between REM Sleep and Memory Consolidation in Old Age and Effects of Cholinergic Medication.” Biological Psychiatry, vol. 61, no. 6, Mar. 2006, pp. 750-757. ScienceDirect, doi: doi.org.ezproxy.cul.columbia.edu/10.1016/j.biopsych.2006.08.034
 Ellenbogen, Hulbert, Stickgold, Dinges, and S. Thompson-Schill. “Interfering with Theories of Sleep and Memory: Sleep, Declarative Memory, and Associative Interference.” Current Biology, vol. 16, no. 13, 11 July 2006, pp. 1290-1294. ScienceDirect, doi : doi.org.ezproxy.cul.columbia.edu/10.1016/j.cub.2006.05.024