Normal sleep progresses through 5 stages that are delineated by the electrical patterns of the brain that occur during each stage of sleep. Typically, these stages are detected through the use of an EEG (electroencephalogram), a medical device which senses electrical activity while a sleep study subject rests.
During the first 4 stages of sleep, the muscles in the eye are relaxing in what is commonly called NREM (non-rapid eye movement) sleep characterized by a relaxed body and an idle brain. The last stage of sleep, Stage 5, consists of increased contraction of the eye muscles and the onset of REM (rapid eye movement) sleep which has been popularized as the time when vivid dreams can occur. However, some researchers have noted the presence of dream activity during NREM sleep as well.
A normal night of sleep has two sleep cycles: active, or Stage 5 sleep, and quiet sleep where the body shifts through Stage 1, 2, 3, and 4. Below, the 5 stages of sleep are outlined with some of their defining characteristics:
Stage 1 Sleep
- Composes approximately 2-5% of a normal night of sleep
- The initial stage into falling asleep
- Typically characterized by NREM sleep
- Eye and muscle activity slows from the waking state
- Sudden muscle contraction (hypnic myoclonia) may occur after the sleeper experiences the sensation of starting to fall
- The sleeper drifts in and out of sleep and can be awakened easily
- No consistency in alpha waves as would be seen in the waking state
- People awakened from this stage will commonly recall only fragmented visual images
Stage 2 Sleep
- Composes approximately 45-55% of a normal night of sleep
- Commonly characterized by NREM sleep
- Follows Stage 1, but is a deeper state of sleep
- Eye movements stop and brain waves slow with occasional rapid bursts of rythmical brain activity called sleep spindles
Stage 3 Sleep
- Combined with Stage 4 composes 10-25% of a normal night of sleep
- Delta waves begin to appear mixed with faster, smaller wave patterns
- Sleep spindles disappear from EEG readings
- No eye movement or muscle activity
- Called a "slow wave" sleep stage because brain activity slows dramatically as the person progresses toward Stage 4
- Termed deep sleep, in which it is very difficult to wake the sleeper. If someone is awoken during this stage they are typically groggy and disoriented for several minutes
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