What Happens to Your Body While You Sleep
Sleep may seem like a fairly uncomplicated thing, especially when it’s going well—you crawl into bed at night, fall asleep in the morning, and wake up feeling refreshed and ready for the day ahead. In reality, you pass through many different stages of sleep through the night in a cycle that repeats about every 90 to 110 minutes. These stages are not all the same, and scientists believe that different types of sleep serve different purposes.
There are two main types of sleep: REM (rapid eye movement), the type of sleep that includes our most vivid dreaming, and NREM (non-rapid eye movement), which makes up about 75 to 80 percent of our sleep and includes periods of restorative rest. NREM sleep is broken down into four stages, which are followed by a period of REM sleep.
These five stages of sleep progress from half-awake sleepiness through deep sleep to dreaming as follows:
Stage 1—lasts 1 to 7 minutes—In this stage you feel drowsy, and your brain activity starts to slow. However, you’re still somewhat alert and can be fully woken up easily at this point.
Stage 2—lasts 10 to 25 minutes—In this stage of light sleep, your brain waves continue to slow and increase in size on an electroencephalogram (EEG) monitor. Interspersed with these readings are spikes of brain activity called “sleep spindles.” You can still be woken up easily in this stage. Your heart and breathing rate become even and your body temperature drops.
Stage 3—lasts 20 to 40 minutes—In this stage you enter deeper sleep, with sleep spindles disappearing from EEG readings and even slower, taller waves known as delta waves starting to appear. The longer that stage three sleep lasts, the harder it is for someone to wake you up.
Stage 4—lasts 20 to 40 minutes—This is the deepest stage of sleep. Your muscles relax, your breathing slows down, and you are extremely hard to wake up. This is the most restorative stage of sleep, when your body repairs muscles, boosts your immune system, stimulates growth, and gathers energy for the day ahead.
REM Sleep—lasts 10 to 60 minutes—This stage is named for the jerky, fast eye movements that occur beneath your closed eyelids. Your heart rate, blood pressure, and breathing rate all increase during this stage, and your brain becomes much more active, resembling the EEG of someone who is awake. However, your brain sends signals that relax and immobilize your body. This stage is associated with vivid dreaming. It is also believed to play an important role in processing information and forming long-term memories from our daytime experiences.
When your sleep is cut short or is frequently interrupted, you may not spend enough time in the restorative stages of sleep, shortchanging both your body and your mind. It’s important not only to schedule enough time for sleep, the way you would schedule any important activity, but to make sure that your sleep environment is conducive to getting good quality rest.
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