What Are the Sleep Stages?
When the lights go out, your body’s sleep cycle is only just beginning. Learn what you need to know about the sleep stages.
Sleeping may seem like a passive activity. However, as you’re peacefully asleep in your bed, your body is cycling through four stages that serve specific purposes. Learn about the different sleep stages and how a normal sleep cycle works.
What Is a Sleep Cycle?
While scientists in the 1950s thought that your brain went into “shutdown” mode while you slept, we now know that your sleeping body cycles through regular sleep patterns of activity, known as the sleep cycle. Over the course of one night, your body goes through the sleep stages every 90 minutes or so. Sleep stages last for different periods of time depending on the age of the sleeper.
The first three sleep stages are categorized as non-REM sleep, and the fourth and final sleep stage is Rapid Eye Movement (REM) sleep.
Stage 1 of the sleep cycle is the lightest phase of sleep and generally lasts about seven minutes. The sleeper is somewhat alert and can be woken up easily. During this stage, the heartbeat and breathing slow down while muscles begin to relax. The brain produces alpha and theta waves.
In Stage 2, the brain creates brief bursts of electrical activity known as “sleep spindles” that create a distinct sawtooth pattern on recordings of brain activity. Eventually, the waves continue to slow down. Stage 2 is still considered a light phase of sleep, but the sleeper is less likely to be awakened. Heart rate and breathing slow down even more, and the body temperature drops. This stage lasts around 25 minutes.
This stage represents the body falling into a deep sleep, where slow wave sleep occurs. The brain produces slower delta waves, and there’s no eye movement or muscle activity from the sleeper. As the brain produces even more delta waves, the sleeper enters an important restorative sleep stage from which it’s difficult to be awakened. This phase of deep sleep is what helps you feel refreshed in the morning. It’s also the phase in which your body repairs muscle and tissue, encourages growth and development, and improves immune function.
About 90 minutes after falling asleep, your body enters REM sleep, which stands for Rapid Eye Movement sleep and is named so for the way your eyes quickly move back and forth behind your eyelids. REM sleep is thought to play a role in central nervous system development in infants, which might explain why infants need more REM sleep than adults. This sleep pattern is characterized by dreaming, since your brain is very active during this stage. Physically, your body experiences faster and irregular breathing, increased heart rate, and increased blood pressure; however, your arm and leg muscles become temporarily paralyzed, stopping you from acting out your dreams. REM sleep increases with each new sleep cycle, starting at about ten minutes during the first cycle and lasting up to an hour in the final cycle. Stage 4 is the last stage before the cycle repeats. This sleep stage is critical for learning, memory, daytime concentration, and your mood.
While all sleep stages are important, Stage 3 and REM sleep have unique benefits. One to two hours of Stage 3 deep sleep per night will keep the average adult feeling restored and healthy. If you’re regularly waking up tired, it could be that you’re not spending enough time in that deep sleep phase. Meanwhile, REM sleep helps your brain consolidate new information and maintain your mood – both critical for daily life. Talk to your health care provider if you feel you are not getting the restful sleep that you need.