You spend about one-third of your life asleep, but what does this really mean?
Most of us are clueless about what happens during shut-eye, but it’s not the passive, do-nothing state you might imagine it to be. Sleep is a highly complex biological process that is necessary for your survival. Learn more about what happens while you sleep to have a better appreciation for the next time you crawl underneath the covers.
An Active Process
Not all sleep was created equal. Sleep is a dynamic process rather than a static process, causing changes in your body’s organs, including the brain. It occurs in two main stages: non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM). Brain waves during NREM sleep tend to be slower, with greater amplitude. Brain waves during REM sleep are fast, desynchronized, and more random, with bursts of eye movement. Some scientists believe that the eye movements might be related to images seen in dreams, but in general these movements are still not fully understood. During REM sleep, your body’s arms and legs rest in a state of near-paralysis. Your body cycles between NREM and REM sleep throughout the night, with each cycle lasting 90 to 110 minutes.
The Four Stages
Although scientists differentiate between two types of sleep (NREM and REM sleep) your body actually goes through four stages during a night’s sleep. The first three stages comprise NREM (N1, N2, N3) sleep; the last stage is REM sleep. In Stage N1, you drift into light sleep. During Stage N2, you become disengaged from your surroundings, your body temperature drops, and your breathing and heart rate become regular. In Stage N3, you enter deep, restorative sleep. Very slow brain waves, called delta waves, emerge in Stage 3. Your tissues grow and repair, hormones are released, and blood supply to your muscles increases. It can be very difficult to wake someone up during Stage N3! During this phase, your energy is restored. In REM sleep, your body and brain continue to experience reenergizing. Your eyes dart back and forth, and you dream.
Changes In Your Body
Your brain stays active during sleep, although neurons tend to fire less randomly compared to periods of wakefulness. Brain activity appears to be a bit more coordinated and synchronous during sleep. Interestingly, our body temperature tends to drop 1 degree to 2 degrees Fahrenheit during sleep. One theory is that the decreased temperature helps us feel sleepy; another theory is that a lower temperature is a good way to conserve energy during sleep. During NREM sleep breathing rates tend to be more regular than during wakefulness; heart rates and blood pressure also drops. Kidney function slows, and urine production decreases. However, in REM sleep, blood pressure, pulse, and respiratory rate can vary dramatically. The cardiovascular system is at its most unstable during this phase of sleep.
National Institutes of Health, Information About Sleep: https://science.education.nih.gov/supplements/nih3/sleep/guide/info-sleep.html
University of Pittsburgh Schools of the Health Services, What Happens During Sleep?: http://www.upmc.com/services/pulmonology/sleepmedicine/resources/pages/during-sleep.aspx
National Sleep Foundation, What Happens When You Sleep?: http://sleepfoundation.org/how-sleep-works/what-happens-when-you-sleep
Harvard Medical School, The Characteristics of Sleep: http://healthysleep.med.harvard.edu/healthy/science/what/characteristics