What is the difference between REM sleep and deep sleep, and do I need both?
You need both sleep stages: deep sleep and REM sleep each do something different for your memory and brain, and getting a full night's sleep is the most concrete way to obtain enough of both.
Deep sleep (also known as slow-wave sleep or N3) dominates the first half of the night. Your brain produces slow, synchronised waves during this time. These waves are associated with movements of cerebrospinal fluid that are linked to the flushing away of metabolic waste products. The causal evidence for this in humans is still limited, but the pattern is clearly different from that of light sleep.
For memory, deep sleep has the strongest evidence: multiple studies show that facts and word pairs are better retained after a night with a lot of deep sleep early in the night. REM sleep, by contrast, dominates the second half of the night, with vivid dreams and an active brain in a relaxed body. REM appears to weave newly stored memories into your existing knowledge, but does so partly by subtly modifying those memories. In research, this effect only became measurable after a week, not immediately after sleep.
The classic idea that REM is exclusively responsible for motor learning and emotional memories is being called into question. More recent research shows that deep sleep is also involved. In people with Alzheimer's disease, both sleep stages are reduced, and the less of each, the more severe the cognitive decline. Whether this is a cause or a consequence is something current research cannot answer.
Deep sleep and REM sleep are also biologically measurably disrupted in severe depression: the first REM period begins earlier in the night and is more active, while deep sleep decreases. This is not an incidental finding but an established biological hallmark of the condition.
In short: you need both, and they do different things. Deep sleep consolidates factual memory and is associated with cleansing processes in the brain. REM sleep integrates memories and plays a role in emotional processing. Because deep sleep occurs mainly early in the night and REM sleep late, getting a full night's sleep is the practical key to obtaining enough of both.
Based on multiple human studies and one meta-analysis (24 studies in Alzheimer's patients). The causal evidence for cerebrospinal fluid clearance in humans is still limited; the memory findings are more robustly supported. All claims can be traced back to PMID 24395522, 37567411, 41055981, 35365609, 9554322, 28575720.