Not all sleep is equal.
Since the 1930s, modern medicine has known that a full night’s sleep is not a single, continuous experience. Based on EEGs (electro encephalograms) of their nocturnal subjects, sleep researchers discovered distinct, measurable changes in both the body and brain, occurring regularly and predictably throughout the night.
Now, nearly a century later, researchers have refined those discoveries into four well-defined sleep stages:
- Stage 1 (N1, or non-REM 1)—Light sleep, no rapid eye movements (REM)
- Stage 2 (N2)—Light sleep, no REM
- Stage 3 (N3)—Deep sleep, no REM (formerly N3 & N4)
- Stage 5: REM Sleep
Together, these four sleep stages make up a single sleep cycle, lasting between 60 and 90 minutes each. People normally experience three or four sleep cycles per night, their bodies automatically advancing through each stage in sequence, and waking naturally after approximately eight hours.
Although nobody knows exactly why most living things sleep, medical research suggests that time spent sleeping is essential for performing essential maintenance and repair within the body.
What are the differences among the four stages of sleep?
Stage 1 (N1) Sleep: Light sleep, non–REM 1
Taking up about five percent of our total sleep time, Stage 1 sleep lasts for about five to ten minutes at a time. In addition to marking the beginning of your sleep period, Stage 1 sleep also occurs between each subsequent stage of sleep. For example, your 90-minute sleep stage map may typically read as follows:
N1 -> N2 -> N1 -> N3 -> N1 -> REM
Stage 1 sleep may be described as light, shallow, or transitional, making it easy to wake up. Your brainwave activity will show primarily a relaxed, quiet head, with occasional blips of alpha waves that characterize an awake, alert mind.
Stage 2 (N2) Sleep: Light Sleep, Non-REM 2
The greatest amount of sleep time—approximately fifty percent of the total—is spent in Stage 2 sleep. The actual time spent in each N2 stage is about 20 minutes, but in this relatively brief period, your body physiology is adjusting downwards in preparation for sliding into the actual restorative later stages of sleep. Physically, the following changes occur:
- Heart rate, breathing, and other body functions slow
- Body temperature drops
- Blood pressure drops
- Brainwaves lengthen (becoming larger)
- "Sleep spindles" emerge; these are peaks of brain activity considered essential in maintaining communications within the brain itself, as well as memory and learning
While classified as “light sleep,” sleepers in the N2 stage are harder to wake up than those still in N1.
Stage 3 (N3) Sleep: Deep Sleep, Non-REM 3
A quarter of our time asleep is spent in Stage 3 (N3), which typically initiates about 40 minutes after first falling asleep. This stage is also called Deep Sleep, or delta sleep, due to the brain’s production of long, slow waves called delta waves.
In previous years, a Stage 4 (N4) was also identified, but as of 2008 has been merged with Stage 3 (N3) into a single stage.
We enjoy complete unconsciousness during this phase of sleep, and are almost entirely oblivious to external stimuli, including light, sound, and movements. Waking is difficult, and should you wake, you feel very disoriented (a condition sometimes known as being “sleep drunk”). It is during this phase that common sleep disorders may occur, particularly in children, including bedwetting, night terrors, and sleepwalking.
Deep sleep is the most physiologically profound stage of sleep. When you enter this stage, your body releases human growth hormone (HGH), a powerful substance that plays a vital role in cellular repair. Built-up waste products are flushed away, tissues are repaired and regrown, bones and muscles are built especially in growing children, and the immune system is strengthened.
Subjectively, deep sleep is considered to be the most refreshing portion of the entire sleep cycle. It effectively erases the accumulated need for sleep that builds over a normal day of wakefulness, and may play a major role in helping clear the brain for new learning the following day.
Your longest periods of rejuvenating deep sleep occur in the first two sleep cycles. With each successive cycle, N3 sleep decreases and is replaced by N2 and REM stage sleep. The amount of deep sleep you get shortens as you age.
Stage 5 Sleep: REM (Rapid-eye Movement)
The final twenty percent or so of our sleep time is spent in Stage 5 REM sleep, known as rapid-eye movement sleep (REM), characterized by the following:
- Rapid movements of the eyes beneath the eyelids
- Body paralysis
- Increase in heartbeat and breathing (respiration), sometime irregular
Among the most important effects of REM sleep are the stimulation of learning, the processing or the day’s experiences and thoughts, and the consolidation of memory into long-term storage.
Experiencing sufficient REM sleep is essential for normal functioning, both sleeping and waking. The symptoms of insufficient REM sleep include mental problems, including impaired memory, hallucinations, mood swings, and inability to concentrate. Physical problems observed include lowered core body temperatures, impaired immune systems, and in extreme cases, death.
The potential consequences are so extreme that the body will do anything to reclaim a REM sleep debt, including forcing a sleep-deprived person into unconsciousness, or sleeping abnormally long periods until the balance is sufficiently restored.
What stage of sleep is the most important?
For a quick review of the four stages of sleep:
- Stage 1 (N1): This is the lightest sleep, lasting as little as five minutes, and the easiest from which to pop back awake. It begins the process of sleeping.
- Stage 2 (N2): This light sleep is also fairly easy to be awoken from, but it is by far the single longest-lasting phase of sleep. This stage prepares the body for deep sleep by slowing metabolic processes.
- Stage 3 (N3; formerly N3 and N4): This is deep sleep, during which the body performs maintenance and repair all the way down to the cellular level. This is the only stage of sleep that is “refreshing,” in that it erases the cumulative sleepiness collected during a normal day of waking activities.
- Stage 5: REM sleep is the only sleep stage in which we dream. This type of sleep is essential for learning, retaining, and editing our memories. Lack of this stage of sleep leads to physical and mental hardships, and could contribute to an early death.
Stages 1 and 2 are light sleeps, and do little to refresh or repair our bodies and minds. Stage 3 is where the powerful HGH hormone is triggered, sending a wave of rejuvenating cellular work throughout the body. But all of this would be for nothing if the brain itself were to become psychologically damaged by the lack of Stage 5 REM sleep.
And the winner is …
It is a very close call between Stage 3 deep sleep and Stage 5 REM sleep. Deep sleep is different from REM sleep in that deep sleep is geared towards physical maintenance, while REM sleep takes care of mental maintenance.
However, since even a physically perfect human body would be of no use if its brain has been driven insane by a lack of REM sleep. . .
Stage 5 REM sleep is hereby crowned the winner of this contest!