The first stage of sleep, the first phase of the NREM cycle, is a transition between wakefulness and sleep. Phase I is characterized by drowsiness, severe relaxation and drifting thoughts. Muscles begin to relax, and one experiences a floating, detached feeling. Relaxation slows the heart and lowers body temperature. Yawning is also characteristic of this phase. Yawning stems from the need for more oxygen, and is known to aid in relaxation. Thoughts are mixed with dreams and brain cells go on standby. However, the drowsy sleeper is still easily roused, startled by sudden muscle jerks and movements, wakened by sudden noises. One is, however, blind during this phase of sleep. Even bright lights can not provoke reactions. This phase lasts only a few minutes at longest.

    Phase II marks the first stage of true sleep in the cycle. This light sleep is a drowsy state, similar to the latest stages of dreaming. Sleepers awakened during phase II are likely to insist they had not yet fallen asleep. Brain activity changes; slow regular brain waves are mixed with short bursts of rapid activity. Thoughts are broken and incomplete; vague dreams and images drift carelessly through the mind. Sleepers can still be easily disturbed during this phase, reacting to both sound and touch stimuli. Random muscle activity may also startle and wake sleepers. Eyes are still unresponsive to light. This phase of sleep lasts seven to ten minutes on average.

    The third and fourth phases of sleep are the most important to your sleep health. These two phases of sleep are the deepest and most restful. People deprived of these restorative stages of sleep can be depressed, listless and sluggish. It is very difficult to arouse sleepers from these stages of deep sleep. Combined, phases III and IV last between fifteen and thirty minutes.

    Phase III is the final transition, leading sleepers into a deep and restful slumber. This step is characterized by increased relaxation. Brain waves become slower, the body is now completely at ease, and the heart rate becomes more stable.

    The fourth phase of sleep is known as delta sleep. Phase IV is the deepest stage of NREM sleep. It is a time of healing and restoration for a tired body. Thinking is fractured as there is another decrease in brain activity. Muscles in the body are more relaxed in delta sleep than when awake. Blood pressure continues to fall during this phase, and the temperature regulation of the body is impaired. Delta sleep can leave sleepers vulnerable to sleep walking, bed-wetting and even sleep talking.

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