During the course of an eight hour sleep period, a healthy sleeper should cycle through the various sleep stages every 90 minutes or so.
Stages of Sleep
Stage N1 (NREM1) sleep is a transition period from being awake to falling into sleep. During this time you may have a sudden dream onset. You are drifting off to sleep and may still feel aware of your surroundings and easily be aroused back into wakefulness.
From Stage N1, you will enter Stage N2 where your breathing and heart rate will begin to slow. During the continuous sleep cycles throughout the night, we should spend about half of our sleep time in Stage N2.
Next comes Stage N3, sometimes referred to as “delta sleep” because of the slow delta brain waves that have been recorded during this sleep stage. Delta sleep is a regenerative period where your body heals and repairs itself. Sometimes during illness, we fall immediately into delta sleep because infection fighting antibodies are produced in greater numbers in this sleep stage. This is an important sleep stage, and explains why doctors recommend we get plenty of sleep when we’re sick.
The first episode of Stage N3 lasts from 45-90 minutes. Subsequent episodes of delta sleep have shorter and shorter time periods as the night progresses. After several complete sleep cycles earlier in the night, the body does not re-enter delta any longer, but enters the 4th Stage also called R (REM).
Stage R is referred to as REM or "rapid eye movement." It is during REM sleep that we dream. Our body creates chemicals that render us temporarily paralyzed so that we do not act out our dreams. In this stage, our mind is extremely active, and our eyes, although closed, dart back-and-forth as if we were awake.
Our heart and breathing rate increases and becomes irregular. As the REM stage ends, our body temperature begins to rise, and breathing and heart rate returns to normal. Most of us will remember dreams from the REM stage.
During a diagnostic overnight sleep study, or polysomnogram, our sleep architecture is tracked and recorded. With over 80 different sleep disorders, often our diagnoses are partially determined by how we cycle through these sleep stages. As an example, people with narcolepsy often skip the first stages and fall directly into REM sleep. People with apnea often don’t reach stage 3 before having an apnea episode and then returning to stage 1 — over and over all night long. If you’re experiencing the symptoms of sleep deprivation but getting enough hours of sleep, a polysomnogram can help identify other problems.
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