What are the stages of sleep?

Your sleep cycle-what are the different stages?

Understanding Your Sleep Cycle

Have you ever stopped thinking that a third of your life has passed asleep? Indeed, sleep takes more time each day than any other activity. But what happens while we sleep?

Most people have heard of rem sleep and non-paradoxical. Paradoxical sleep, or sleep with rapid eye movements, is the period in which we dream. The rest of the sleep cycle consists of non-REM sleep.

Non-REM sleep is divided into four necessary steps. We go through these stages throughout the night, each cycle leading to a period of REM sleep. The cycle lasts between 90 and 110 minutes, after which it repeats.

Early in the night, our body needs a deep sleep, which means we spend more of our sleep cycle during the phases of deep sleep. As the night progresses, we are more rested and spend more time in REM sleep.

To understand the stages of sleep, it is essential to know how scientists measure the sleep cycle. The first measurement is made with an electroencephalogram or EEG. The EEG measures brain waves while sleeping. Each stage is characterized by a different pattern of brain waves. The next measure is muscle tone, which is taken with an electromyogram, or EMG. This measures the effect of different stages of sleep on the muscles. Finally, eye movement is measured using an electrooculogram, or EOG.


 When you wake up, your brain is very active. Your brain waves have a high frequency, which means that on a graph they occur close to each other, and of low amplitude, which means that on a chart they are short, without large spikes. Brain waves do not follow a regular pattern but change continuously as you progress through your day. These brain waves are called beta waves.

When you relax, your brain waves become more regular. The amplitude is increasing, and the frequency is slowing down. These brain waves are called Alpha waves.

Step 1

 The first phase of sleep occurs when you go to bed. You enter a relaxing state in which you are mostly asleep, but are always easy to wake up. Your eyes will move slowly as you drift into a semi-conscious state. Sometimes sleepers "start to sleep" when sudden muscle contractions, called hypnetic myoclonus or myoclonic tremors, cause a sensation of falling. As you drift in sleep, your brain waves slow down even more than during relaxation, floating in a slower frequency, a wave of higher amplitude called a theta wave. Stage 1 sleep does not usually last very long.

Step 2

 Sleep stage 2 is more profound than that of step 1. Your eyes stop moving and your brain waves slow. Occasional bursts of rapid brain activity called sleep spindles, and periods of higher wave amplitude, called K-complexes, are typical of Stage 2 sleep. Most brain waves are theta waves in stage 2 sleep. As sleep stage 1, sleep Stage 2 does not last more than a few minutes.

Step 3

As you drift deeper into sleep, your brain will settle into a slow pattern with a high amplitude called Delta waves. It's the beginning of deep sleep. Your body is restful, your eyes are still, and you are deeply asleep. In Stage 3 of sleep, just under 50% of your brain waves are Delta waves, with higher activity peaks between quieter periods.

Step 4

 Like Stage 3, this stage is characterized by the brain activity of the Delta wave. More than 50% of your brain waves are Delta waves, but occasional flashes of higher activity still occur. It is tough to awaken a person from stage 4 sleep. It is interesting to note that it is during Stage 4 sleep that most cases of sleepwalking, night terrors and even night incontinence occur. Stage 4 sleep lasts the longest at the beginning of the night but gradually decreases as the night progresses until it disappears almost early in the morning.


Once the body has moved from the first to the fourth stage of sleep, it will reverse the pattern, returning to the early stage of sleep just before entering paradoxical sleep. As you begin to dream there are many changes in your body. Your breathing becomes rapid and irregular, your eyes start to move, often with jerky movement, and your heart rate becomes high. At the same time, your brain waves become active, much like those you found when you were awake. The body produces a chemical that paralyzes the muscles while you dream, to prevent you from fighting and hurting yourself. This is measured by EMG, which takes a sudden and drastic loss of muscle tone. If you are awake during REM sleep, you will probably remember your dream in detail.

Most people will go through this cycle of sleep stages 3-5 times throughout the night. Early stages do have long periods of REM sleep, but as the night progresses, your stage dream will lengthen. It has been theorized that we use paradoxical sleep to process information taken during the day. It is interesting to note that infants spend 50% of their REM sleep time. As we get older, we need less REM sleep. Adults spend about 20% of the night in rem sleep

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