Sleep Cycles and Your Body

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Scientists are still figuring out exactly why humans need sleep, but they’ve figured out a lot about how it happens. All night long, your body and brain cycle through several different kinds of sleep.

Stage 1

Stage 1, or “light sleep,” happens right as you drift off. It lasts only 5–10 minutes. If you wake up now, you may feel like you haven’t slept at all.

Brain waves slow down.

Heart rate goes down, possibly so your heart muscle can rest. Blood pressure lowers, too.

Breathing slows to a regular, even pattern.

Muscles relax, aside from some occasional twitches that can make you feel like you’re falling (and sometimes jerk you awake)

Up to 70% of people have involuntary muscle twitches, called hypnic jerks, when falling asleep.

"Each night, when I go to sleep, I die. And the next morning, when I wake up, I am reborn.”

Mahatma Gandhi

Stage 2

You spend most of a night’s sleep in this stage. You’re still in “light sleep,” but you become less aware of the world around you.

Heart and breathing rates go down even more.

Muscles get more relaxed.

Eye movements stop.

Body temperature starts to drop.

Your brain waves are still moving slowly, but with short bursts of activity called sleep spindles. Experts think these may be tied to how your brain stores memories.

Just after you learn a new task, you’re likely to have more sleep spindles during sleep.

"Sleep is a lovely hint of oblivion."

D.H. Lawrence

Stage 3

The deepest, most refreshing sleep of the night happens in this stage. During your first sleep cycle of the night, you spend 20–40 minutes in stage 3. It gets shorter or disappears in the cycles after that.

Heartbeat and breathing drop to their lowest levels.

You build bone.

Tissues grow and repair themselves. Your immune system gets a boost.

Brain waves get even slower than they were in stages 1 and 2. That’s why this stage is also called slow-wave sleep.

In Germanic folklore, a mara or mart is an evil spirit that sits on your chest while you sleep, blocking your breath and turning your dreams into nightmares.

REM Sleep

This stage gets its name from the quick, back-and-forth movements of your eyes that happen during it.

It’s also when you dream.

Most people reach REM about 90 minutes after falling asleep. At first, it may only last 5–10 minutes, but REM sleep gets longer in later cycles as the night goes on, sometimes up to an hour.

Eyes move beneath your closed lids.

The brain is more active, and acts more like it does when you’re awake.

Breathing speeds up and gets less regular.

Heart rate and blood pressure go up to almost waking levels.

Body temperature goes down -- it’s the lowest of the night during REM.

Adults have an average of five to six REM cycles during the night.
You can dream during any stage, but you’re most likely to do it in REM sleep.

REM sleep is when dreams and nightmares are most vivid. One reason may be that more blood flows to the limbic system of your brain -- the part that controls your emotions -- and also the areas involved in vision.

Studies show that the movements of your eyes are tied to the spikes in your brain activity. Scientists say this may mean you’re “watching” your dreams as they happen.

Your brain temporarily paralyzes your muscles in REM sleep, which keeps you from acting out the dreams you’re seeing.

Some experts think dreams may play a part in forming memories by replaying parts of your day so you can store them away. Both REM and non-REM sleep are important for strengthening your powers of recall.