9781422273890

In addition to EEGs, another technique for studying sleep involves recording movements of the eyes beneath slumbering eyelids. This study of eye movement during sleep provided the names of the two main types of sleep: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. We cycle back and forth between these states during the night—four or five times on a normal night. Each type of sleep is important for different reasons. Having a better understanding of the sleep cycle may help you make better decisions about how to get the amount of rest that your brain and body need. Non-REM Sleep The first stage of a sleep cycle is NREM sleep. People who study sleep divide NREM sleep into three phases: N1, N2, and N3. (Some descriptions of the sleep cycle add an additional phase to the beginning, before the first N1 stage. It’s essentially when the person begins to feel drowsy. Muscles gradually relax, the heart rate and breathing slow down, and the eyelids get heavier. But this extra phase only occurs when the person first goes to bed—this pre-sleep phase is not part of the sleep cycle that repeats itself throughout the night. That’s why not all descriptions of the sleep cycle include it.) NREM sleep is progressive , in the sense that each of the N1, N2, and N3 phases is a “deeper” sleep than the one before. Breathing and heart rate slow down, as does brain activity. The N3 phase is sometimes called the “slow wave” phase because of the way brain waves look on an EEG. In the earliest part of the cycle, it’s fairly easy to wake someone up. But as NREM progresses, that gets harder—someone awakened from N3 sleep is likely to be groggy and confused. The entire NREM

Understanding Sleep

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