How does Sleep work?
Sleep is a complex process that involves several stages and cycles. It is regulated by both the circadian rhythm and homeostatic mechanisms. When we sleep, our brain goes through different stages of activity, including non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep consists of four stages characterized by progressively deeper levels of relaxation, while REM sleep is associated with dreaming and increased brain activity. Throughout the night, we cycle through these stages multiple times. Various neurotransmitters, such as melatonin and adenosine, play a role in regulating our sleep-wake cycle.
Long answer
Sleep is an essential physiological process that allows us to rest and restore our body and mind. It incorporates a complex interplay between biological rhythms, neuronal networks, hormones, and environmental factors.
The regulation of sleep is governed by two main processes: the circadian rhythm and homeostasis. The circadian rhythm controls our sleep-wake cycle over a 24-hour period. This internal biological clock is influenced primarily by the suprachiasmatic nucleus (SCN) in the hypothalamus, which receives light signals from the eyes to synchronize with external day-night cycles. Melatonin release from the pineal gland also follows this pattern and helps promote drowsiness.
Homeostasis regulates the drive to sleep based on prior wakefulness or sleep duration. Adenosine, a molecule that builds up during wakefulness as a byproduct of energy metabolism, accumulates in certain brain regions over time. Adenosine promotes sleepiness by inhibiting wake-promoting areas in the brain.
Sleep itself involves distinct stages grouped into two main types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep accounts for about 75% of total sleep time and has four sequentially numbered stages (N1, N2, N3, N4), which progressively become deeper. During N1, a transition from wakefulness to sleep occurs, and muscle activity reduces. In N2, brain waves slow down, and sleep spindles and K-complexes appear. N3 is characterized by slow-wave sleep (SWS) with large, synchronized delta waves. Finally, N4 represents the deepest stage of sleep.
REM sleep is distinct from NREM sleep and occurs periodically throughout the night. It is characterized by rapid eye movements, heightened brain activity similar to wakefulness, vivid dreaming, muscle paralysis (atonia), and irregular heart rate and respiration. REM sleep is essential for cognitive processes such as memory consolidation and emotional regulation.
Throughout the night, we go through cycles of these stages that last around 90-120 minutes on average. As the night progresses, the proportion of REM sleep increases while deep NREM sleep decreases.
Overall, sleep remains a fascinating area of study with many unanswered questions. Nonetheless, decades of research have shed light on its various physiological mechanisms and highlighted its vital role in maintaining our overall health and well-being.