Sleep takes up nearly one-third of a person’s life, yet many still wonder what really happens while we rest. Until the mid-20th century, scientists believed sleep was simply a time when the body and brain shut down. Research now shows that sleep is far from passive, it’s an active, restorative process essential for mental and physical health. As Johns Hopkins neurologist and sleep expert Dr. Mark Wu explains, the brain remains deeply engaged during sleep, performing vital tasks that influence memory, mood, and overall well-being.The Stages of SleepExperts say the human sleep cycle has four main stages that repeat throughout the night. The first three make up non-rapid eye movement (non-REM) sleep, while the fourth is REM sleep, the stage most closely linked with dreaming.In the first stage of non-REM sleep, the brain and body begin to shift from wakefulness to rest. Brain activity slows, muscles relax, and it is common to experience small, sudden twitches.During the second stage, the body’s temperature drops slightly, and breathing and heart rate slow. Brainwaves continue to decelerate, though quick bursts of activity may still appear as the brain processes and stores information.The third stage marks deep sleep, which is the most restorative phase. Here, the body fully relaxes, and the heart rate, breathing, and brain activity reach their lowest levels. This stage is crucial for waking up feeling refreshed and for healing and repair processes throughout the body.The final stage is REM sleep, which begins about 90 minutes after you fall asleep. It starts short, roughly 10 minutes, but lasts longer with each cycle. During REM, the eyes move rapidly beneath the eyelids, breathing quickens, and heart rate and blood pressure rise to near waking levels. This is when most dreaming occurs. Interestingly, as people age, the amount of REM sleep they experience gradually decreases.How The Body Regulates SleepAccording to Dr. Wu, two main forces govern sleep: the circadian rhythm and the body’s sleep drive.The circadian rhythm acts as the body’s internal clock, controlled by a cluster of brain cells that respond to light and darkness. This rhythm triggers the release of melatonin at night and halts it when morning light appears. People who are completely blind often struggle with sleep because their brains can’t register these light cues properly, as per the John Hopkins Study.The sleep drive works much like hunger. The longer you stay awake, the stronger your urge to sleep becomes. Unlike hunger, though, your body can override your willpower, if exhaustion sets in, it can force sleep to happen, even during daily activities or while driving. In extreme fatigue, brief “microsleep” moments lasting just a few seconds can occur without a person realizing it. However, taking long naps later in the day can reduce this natural sleep pressure, making it harder to fall asleep at night.Why Sleep Matters For Your BrainAnyone who has felt mentally sluggish after a sleepless night knows how strongly rest affects the brain. Adequate sleep is key to brain plasticity—the ability to learn, adapt, and form memories. Without it, the brain struggles to retain new information and perform cognitive tasks. Scientists also believe that deep sleep allows the brain to clear out toxins that build up during waking hours, improving long-term brain health.Sleep impacts far more than the mind. Poor sleep can worsen conditions like depression, high blood pressure, migraines, and even seizures. It weakens the immune system, leaving the body more vulnerable to infection. Metabolism also suffers, as just one night without enough rest can temporarily throw the body into a prediabetic state.As Dr. Wu explains, “There are countless ways sleep supports health.” From mental clarity to physical repair, the hours we spend asleep are some of the most important for keeping the body and brain functioning at their best.
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