You know that sleep is essential for your overall wellbeing. (Just try to make it through a workday on too little shut-eye.) But how much do you actually know about what happens to your mind and body while you're fast asleep? It's time to take a trip back to science class.

The stages of sleep: an overview

There are five stages of sleep, each of which lasts about five to 15 minutes. According to the National Sleep Foundation (NSF), sleep follows a specific pattern of alternating REM (rapid eye movement) and NREM (non-rapid eye movement) sleep throughout the night.

The pattern repeats itself about every 90 minutes. NREM sleep is comprised of stages 1 through 4 and accounts for about 75% of adult sleep, while the fifth stage, REM sleep, accounts for 25%. (Babies spend about 50% of their sleep in the REM stage.)

Each sleep stage serves a different function.

Stage 1

The first stage of sleep is light sleep. During this stage, you drift in and out of sleep, your eyes move slowly, and muscle activity starts to wind down. It's very easy to be awakened when you're in the first stage of sleep. This is also when you might experience the feeling of falling that can cause sudden muscle contractions, a phenomenon known as hypnic myoclonia. If you're woken up during this sleep stage, you might feel like you haven't slept at all.

Stage 2

This is also considered a light sleep stage. During the second stage of sleep, your body starts to prepare itself for the upcoming deep sleep stages. Eye movement stops, brain waves slow, body temperature starts to drop, and heart rate slows down as well.

Stages 3 and 4: deep sleep

The third stage of sleep marks the transition from light to deep sleep. Stages three and four of sleep are characterized by slow delta brain waves. If you're woken up during a deep sleep stage, you might feel disoriented for a while—but it might actually be very difficult to be woken up at all in this point in your sleep cycle.

It's in these stages of sleep that people experience parasomnias (disruptive sleep disorders), such as sleepwalking. That's because parasomnias tend to occur when you're transitioning from non-REM to REM sleep.

Deep sleep is also known as slow-wave sleep. It's the deepest and most restorative sleep you'll experience during the night. Deep sleep is crucial for refreshing your mind and body for the next day. Tissue repair happens during deep sleep, as does the release of hormones, such as growth hormone (which is needed for growth and development). Older adults have trouble entering a state of deep sleep—they also tend to sleep less and wake up more often.

Stage 5: REM sleep

This sleep stage usually occurs after the first 90 minutes of sleep. The first instance of REM sleep usually lasts around 10 minutes, and each subsequent REM sleep period throughout the night gets longer. Your final period of REM sleep will last about an hour. During this stage of sleep, your eyes stay closed, but they move super-quickly from side to side.

You also experience increased brain activity at this point in the sleep cycle, which is why dreams most commonly take place in REM sleep. You may dream anywhere from four to six times a night. (Here are common causes of crazy dreams.) Your body relaxes in REM sleep and your muscles stop moving. This paralysis, explains The Better Sleep Council, is a protective measure to stop you from acting out your dreams.

Next, learn all about lucid dreaming and what you can do to control what happens in your dreams.