The two main phases of sleep might work together to boost creative problem-solving.
As you start to fall asleep, you enter non-REM sleep. That includes a light phase that takes up most of the night, and a period of much heavier slumber called slow-wave sleep, or SWS, when millions of neurons fire simultaneously and strongly, like a cellular Greek chorus.
The other phase of sleep—REM, which stands for rapid eye movement—is very different. That Greek chorus of neurons that sang so synchronously during non-REM sleep descends into a cacophonous din, as various parts of the neocortex become activated, seemingly at random. Meanwhile, a chemical called acetylcholine—the same one that Loewi identified in his sleep-inspired work—floods the brain, disrupting the connection between the hippocampus and the neocortex, and placing both in an especially flexible state, where connections between neurons can be more easily formed, strengthened, or weakened.