This phenomenon we call “sleep” is something we are all very familiar with, but do you know how it works? Every night, our brain and our body shuts off functions temporarily to refuel, rejuvenate and prepare for the next day. The Sleep Cycle consists of stages that are repeated multiple times during the night.
During sleep, the body moves through five different stages of both REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. Over the course of the night, the body will go through this five-stage cycle four to six times, spending an average of 90 minutes in each stage.
Each of these stages of sleep serve a unique restorative function; including hormone regulation, muscle recovery and memory consolidation, making it very important to have enough time to go through all sleep stages. Without a full night of sleep, your body and mind are deprived of the essential elements needed to help you go through the next day.
Stage 1 of sleep, also known as the transitional phase, occurs when an individual finds themselves floating in and out of consciousness. During this NREM stage, you might be partially awake while your mind will begin to drift off. This period of drowsiness eventually leads to a light sleep. This is also the time when the muscles sometimes jerk, followed by a falling sensation that jolts you back into consciousness. This experience is known as Hypnic Myoclonia. After winding down in stage 1, your sleep cycle will slip you into stage 2.
Almost 50% of the time spent asleep over the course of the night is spent in stage 2. This is the stage where non-REM phase and is one of the lighter stages of sleep where the heart rate begins to slow and the core body temperature decreases. During this stage, eye movement stops and brain waves slow with the occasional burst of waves called Sleep Spindles. Stage two can also be characterized by the unstructured periods that alternate between muscle tone and muscle relaxation.
STAGES 3 & 4
Stages 3 and 4 are characterized as the Deep Stages of Sleep, and are often the hardest to wake up from. If you try to wake someone up when they are in stages three or four, they will most likely be disoriented and groggy for minutes after they awake. Stages 3 and 4 are often grouped together because they are the periods of Slow Wave Sleep (SWS).
Slow Wave Sleep is a NREM phase of sleep, and is the deepest sleep that your body enters throughout the night. It is called Slow Wave Sleep because the brain waves slow to what are known as delta waves with the occasional faster wave.
In addition to the deep sleep caused by the delta waves, blood pressure drops even further, and breathing becomes deeper, slower, and more rhythmic. During slow wave sleep there is no eye movement, and the body becomes immobile. However, even though there is no muscle movement, the muscles still have the ability to function. These are the stages when children sometimes experience nightmares, bedwetting and sleepwalking.
Stages 3 and 4 of sleep are extremely rejuvenating to the body. During this phase of Slow Wave Sleep, hormones are released that aid in both growth and appetite control. The growth hormones help to replenish muscles and tissues that were exerted over the course of the day, and the appetite controlling hormones help limit feelings of excessive hunger the following day.
These hormones are essential to the development of a strong body and help control unnecessary over-eating. In addition to the release of critical hormones, the blood flow to the muscles also increases, providing restorative oxygen and nutrients.
Stage 5 is the only stage of Rapid Eye Movement (REM), and is unlike any other sleep phase because the brain is bursting with activity. Most adults spend about 20% of sleep in REM, while infants spend almost 50%. During non-REM sleep, the mind rests while the body heals, but in REM sleep the mind energizes itself while the body stays immobile.
It is called Rapid Eye Movement because the eyes dart in various directions while the limbs and muscles are temporarily paralyzed. Breathing becomes shallower and irregular while the heart rate and blood pressure rise. Most dreaming takes place in stage 5 as a result of heightened, desynchronized brain waves, almost similar to being awake. This stage of sleep revitalizes the brain, supporting sharp and alert daytime function. Individuals begin waking up at the end of stage 5.
Understanding how Sleep Cycles work is important in order to maintain a healthy mind and body. By explaining how our bodies function, we realize its potential and how well to manage the quantity and quality of sleep.