The five stages of sleep were first identified by Alfred Lee Loomis and his colleagues in 1937, using EEG brain waves. The group used the letters A thru E to distinguish between the different states of brain activity, equaling 5 stages of sleep.
Nearly two decades later, REM or rapid eye movement, was discovered and became classified as a distinct stage, separate from NREM or non-REM states. From the 1950s to the present day, the two primary categories of sleeping brain states are NREM and REM.
NREM and REM are further broken down into other categories. At first, there were believed to be four distinct categories of NREM (N1, N2, N3 and N4), but in 2007, N3 and N4 were combined into just one stage of deep sleep.
REM is now said to consist of two stages; tonic and phasic. So, depending on how scientists look at it, they might refer to 2, 3, 4 or 5 stages of sleep. If N3 and N4 are considered distinct states, then 6 separate states might be recognized.
When we are awake the brain emits a number of different waves, caused by electromagnetic activity. The same is true when we are asleep, but during some phases, specific waves dominate. Scientists categorize the type of sleep that is occurring by analyzing those waves.
When a person is relaxed with their eyes closed, alpha waves dominate. As he or she begins to fall asleep, the electromagnetic activity changes. In brief, here’s what happens.
Each of the five stages of sleep is believed to be important, but researchers are still investigating their importance. Memory storage occurs during both REM and non-REM states. People that do not enter REM, because of brain damage, do not necessarily have poor memories. Researchers believe that it is specific kinds of memories that are stored during each phase.
In normal, healthy individuals, each of the stages may be repeated several times throughout a 7-8 hour night. Typically, people experience 4-5 periods of REM sleep per night, which are often followed by short periods of very light sleep, a “repeat” of the N1 or N2 phases. The deeper N3 and N4 states begin again and may be followed by another period of REM.
Interruption of any of the five stages of sleep or a lack of sleep in general leads to what researchers call a “rebound”. For example, if volunteers are consistently wakened during the REM phase, they will experience longer REM phases during their next sleep cycle.
All mammals and birds are believed to experience the five stages of sleep. Although the importance may not be fully understood, we do know that lack of REM leads to poor immune system function. So, if you don’t get enough REM, you are more likely to get sick.