Stages of Sleep

Going to sleep seems like a pretty straightforward process. You get tired, you power down, climb in bed, and fall asleep. As far as you’re concerned, nothing much happens while you’re sleeping. Maybe a dream or two, maybe a bathroom break, but the hours spent sleeping are pretty uneventful until you wake up the next morning.

So, the occasional late night of socializing, TV binging, or cramming for a final exam, may not seem like such a big deal. You’ll be tired and cranky the next day, but you’ll catch up the next night, right?

Hold on, Night Owl. While you are sleeping, your body and brain are busy processing the previous day and getting ready for the day ahead. In fact, if you get your recommended seven to nine hours of sleep, your brain cycles through four distinct stages of sleep five or six times before you wake up. Each stage of sleep has its own characteristics and function, although not all functions are known yet. Researchers have defined these sleep stages by identifying the brainwave activity that occurs at each stage using an electroencephalogram (EEG).

Miss any of the required number of sleep stages, and you could be looking at a really ugly day ahead. And that’s just one of the many reasons why getting your required hours of sleep is so important.

Anatomy of a Sleep Cycle

Sleep is divided into two phases: Non-Rapid Eye Movement Sleep (N-REM Sleep) and Rapid Eye Movement Sleep (REM Sleep). Experts break NREM sleep into three stages and refer to REM sleep as Stage 4 which completes one sleep cycle.

Each sleep cycle lasts about 90 minutes, but earlier cycles last a bit shorter and later cycles last a bit longer. Curiously, the duration of each stage of the cycle changes as the night progresses, for reasons researchers are still trying to determine. (See Changes in the Sleep Cycle below.)

Stage N1

The sleep cycle begins with this light sleep stage during which you can be easily woken. As you begin to relax, feel drowsy, and fall asleep, your brain begins to slow down and produce slower alpha and theta waves.

During this stage, you are not quite asleep, and you may experience vivid sensations called hypnagogic hallucinations. That’s that weird feeling of falling you sometimes have as you’re drifting off to sleep. Or you might also think you hear someone calling your name during a hypnagogic hallucination. And that sudden startle that jerks you awake for no apparent reason? That’s called a hypnic jerk. This stage of sleep is relatively brief lasting up to seven minutes.

Stage N2

This is also a light stage of sleep, but it becomes a little harder to wake you up. Your eye movements stop, your heart rate slows, and your body temperature decreases. Stage N2 sleep is characterized by sudden bursts of rapid brainwaves called sleep spindles, the exact function of which remains a mystery. Slower brainwaves resume after these brief bursts of brain activity. N2 lasts about 20 minutes per sleep cycle, but for most people, N2 comprises 40% to 60% of adult sleep time.

Stage N3

Also called Slow-Wave Sleep (SWS), this sleep stage marks the beginning of deep sleep during which the brain produces slower delta waves and you become less responsive to external stimuli and are much harder to awaken.

During the latter part of N3 (sometimes known as N4) your brain produces more delta waves, and you fall into a deeper stage of sleep from which it is much harder to be awakened. That’s because some important restorative processes are occurring. During N3, your body repairs muscles and tissues, stimulates growth and development, boosts immune function, and stores energy for the next day. Phew!

Stage R

The first REM Sleep stage occurs about 70 to 90 minutes after you fall asleep and lasts about 10 minutes. As the night progresses, REM Sleep lasts longer until the final one that may extend up to an hour.

During the previous sleep stages, your brain powers down gradually until it falls into Slow-Wave Sleep or deep sleep. However, things change dramatically during Rapid Eye Movement or REM Sleep. Your brain suddenly becomes more active, producing brainwaves similar to those it produces when you are awake and toggling between beta and theta waves.

Related: REM rebound sleep

Most dreams occur during this stage, and they are more vivid than the few dreams that may occur earlier in the sleep cycle. Your eyes move rapidly under your eyelids perhaps in response to your dreams, your heart rate and blood pressure increase, and your breathing becomes fast, irregular, and shallow.

Oddly, your voluntary muscles are temporarily paralyzed during this sleep stage. Experts believe that this paralysis may result from a cool evolutionary trick to protect humans from acting out their dreams and hurting themselves. REM Sleep is believed to be important in long-term memory consolidation and mood regulation.

Changes in the Sleep Cycle

The sum total of your night’s sleep is actually a complex choreography of alternating brainwaves and variously timed sleep stages. As you progress through the night, the amount of time you spend in each stage of sleep changes. You tend to experience more NREM Sleep in the first half of the night shifting to more REM Sleep in the second half. That’s why you often wake from a dream or a series of dreams in the morning.

Your age also determines how much of your night you spend in NREM Sleep and REM Sleep. Babies spend about half of their total period of sleep in REM Sleep. By two years old, they cut that time in half and replace it with deep sleep (N3). By adolescence, 40% of deep sleep is replaced by N2 Sleep where it will remain as you age.

Related: Fragmented sleep

Not only does deep sleep decrease through childhood, but as people get older, the time they spend in REM Sleep will also decrease. By the time people are 70 years old, they spend most of their night in N2 Sleep, which may account, at least in part, for their frequent wakings through the night.

The Purpose of Sleep Stages

Scientists are still learning about the exact role sleep plays in our lives and the exact functions of each sleep stage. But the complexity and sophistication of the way our brain divides up our sleep make it clear that something very important is going on while we’re snoozing. And your body is pretty determined to get what it needs even if you try to deny it.

For example, after a period of partial or total sleep deprivation, your brain will prioritize SWS at the expense of other stages and repay your debt to this stage of sleep in proportion to your sleep loss.