Sleep Spindles

As you may know, there is REM (rapid eye movement) and Non-REM sleep. Typically you begin with NonREM sleep, which has been described as having three stages. In Stage 1 you can be easily awakened. During Stage 2 of, EEG recordings tend to show characteristic sleep spindles; brief bursts of rapid activity in the brain that appear something like the shape of an ‘eye’ as they quickly increase in amplitude and then also quickly decay. Since stage 2 sleep comprises roughly half of a person's sleep, spindles make up a major part of one’s sleep pattern. Sleep spindles are unique to Non-REM sleep. The most spindle activity occurs at the beginning and the end of Non-REM sleep periods. Interestingly enough, studies are showing that some very important brain activities occur when sleep spindles are occurring. Stage 3 of non-REM sleep involves very deep sleep. You are likely to experience some disorientation if you get awakened when in Stage 3. About 90 minutes after you fall asleep you will go into a period of REM sleep, followed by another cycle of Non-REM sleep, and so on. More tomorrow.

https://en.wikipedia.org/wiki/Sleep_spindle