The Different Sleep Stages: What Happens in Shut-eye?
Sleep is one of the most misunderstood human processes. Apart from the common notion that sleep is a state wherein the mind and the body becomes inactive, years of research reveal that sleep in itself is actually a very active process. In fact, the whole process of sleep can be subdivided into two major sleep stages – Non-REM (Rapid Eye Movement) period and the REM Period.
The Two Major Sleep Stages
The two major sleep stages, or sometimes referred to as the levels of sleep, occur in a cyclic and systematic process. Firstly, the Non-REM period takes place. This particular stage of sleep can be further subdivided into 4 stages; stages 1-4, with stages 3 and 4 considered as “deep sleep”.
After the Non-REM period, which is about 90 minutes of sleep, the REM period ascends. It is at this particular stage where dreams most likely appear and are more vivid. After the REM period, the cycle goes back to stage 1 of Non-REM.
REM (Rapid Eye Movement) period was named as such because of the significant discovery of Nathaniel Kleitman and Eugene Aserinsky at the University of Chicago in 1953.
The two were able to observe that when infants sleep, their eyes occasionally move in a very rapid motion for a certain period in the whole sleep cycle. Eventually, they also tried observing adults and – the same observation was noticeable. These movements can be described as scanning ocular motions like that of a person who is awake. Oftentimes, when Kleitman and Aserinsky wake these people up, the subjects claim that they are actually dreaming. Thus, REM period is associated with the stage where dreams occur (Aserinsky & Kleitman, 1953).
How the Sleep Stages are Measured
The different sleep stages can be distinguished from each other using a set of measurement tools. Sleep researchers rely on three basic tools to determine a specific stage as follows:
Electroencephalogram – measures brain wave activity and identifies alpha, beta, theta, and delta waves from each other.
Electromyogram – measures muscle tone.
Electo-oculogram – records eye movement.
The Non-REM period
It is in the Non-REM period wherein bodily functions are in the restorative state; breathing becomes slow, as well as the heart rate, blood pressure and brain activity. It is also in this period wherein hormones are released to help the body in repairing itself. As mentioned, the Non-REM period is subdivided into four sleep stages as follows:
Stage 1 – brain activity is characterized by Theta waves (slightly slower frequency and still, low amplitude). In this stage, the person is in a “light sleep” and he or she is easily awakened.
Stage 2 – Theta waves are still evident. In this stage, sleep spindles and K complexes occur; a sudden increase in wave frequency and a sudden increase in wave amplitude, respectively.
Stage 3 – there is a 20% increase in delta waves (high amplitude, low frequency).
Stage 4 – there is a dramatic, 50% increase in delta waves. The person falls into a “deep sleep” and is difficult to awaken. This is the stage where strange sleep occurrences are evident such as sleepwalking, sleep talking, and night terrors. After this phase, the sleeper nods off to the REM period.
The REM Period
In the REM period, EMG records show a sudden loss of muscle tone, EOG records show an increase in ocular movements and EEG may show a combination of brain waves like as if the person is awake – alpha, beta, and desynchronized brain waves. It is also in this period wherein the sleeper is occupied by vivid dreams.
In a normal slumber, these sleep stages undergo a unique cycle starting from stage 1 of Non-REM and advancing deeper to the REM period (occurs for 10-15 minutes). After the second cycle, the sleeper may enter into the REM period without being on the 4th stage of NREM. Each sleep cycle occurs around 60-90 minutes with 5-6 cycles a night.
Indeed, sleeping is not simply a state wherein the whole body “shuts-off”. Though physically, a person is not moving vigorously during sleep, certain studies claim that biologic processes and brain activity increase while the sleeper goes deeper into the sleep stages – like as if one is just awake.