Although we typically think of sleeping as an opportunity for our bodies to rest, there is a significant amount of neurological activity controlling the various cycles of sleep and the transitions from one stage to another. Each sleep stage is characterized by a specific type of brain activity that usually results from the release of a particular neurotransmitter. The process of falling asleep, as well as staying asleep, is actually quite complex.
Scientists can differentiate between sleep stages based on obvious changes in the neural firing codes. During the early and lighter stages of the sleep cycle, brain activity is sporadic and contains a mixture of fast and slow frequencies with various amplitudes. As sleep becomes deeper, the faster firing waves decrease in amplitude, and slow frequency waves begin to increase in amplitude. The rapid eye movement stage, also known as REM, follows stages one through four. During this period, the body experiences fast twitching of the eyes in addition to increased heart rate and blood pressure. This is the stage of the sleep cycle where vivid imagery and dreams are experienced. In a typical night, most people transition through the entire sleep cycle multiple times, with the longest periods of deeper sleep happening earlier in the evening. By morning, we spent relatively little, if any, time in the deeper cycles.
Although the brainstem limits most physical movements of the periphery during sleep, there are still plenty of vital cellular functions taking place throughout the body. While the organ systems tend to slow down in activity while sleeping, growth hormones trigger cellular expansion, reproduction and repair. Overall heart rate, metabolism, and general kidney function tend to decrease while we sleep, but the constituent cells are hard at work making the necessary replacements for aging cells, and repairing those that are salvageable.
Although the majority of our body is truly at rest during sleep, there is a very complicated pattern of activity that governs the intensity and duration of the individual stages. Not to mention there is also some very complex activity going on in the cellular foundation to maintain optimal functioning while we are awake.