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Sleep Part 2: What are the phases of sleep through life.

By Emilie "Flo" Hester, CPT and Larry Smith, MD, CPT

What is sleep?

That is a question people have been asking for millenniums. Throw in a few dreams and it becomes a mystical component of human existence. A reach into another realm or reality. But the truth be known, sleep and dreams are not unique to humans. Many animals also dream. We've all seen our cats and dogs act out a dream while sleeping. Much of our understanding of sleep has come from research conducted on animals. Is it a direct correlation to humans, probably not. But it is insight into what it is. So, its not unique to humans and not really a mystical experience as much as a necessary biologic one. Definitions of sleep include things like,

Sleep is a recurring, reversible neuro-behavioral state of relative perceptual disengagement from and unresponsiveness to the environment. Sleep is typically accompanied (in humans) by postural recumbence, behavioral quiescence, and closed eyes.”(1)


“Sleep and wakefulness are endogenous, recurring, behavioral states that reflect coordinated changes in the dynamic functional organization of the brain and that optimize physiology, behavior, and health. Homeostatic and circadian processes regulate the propensity for wakefulness and sleep.”(2)

Well to say the least..... Duh.... When there are definitions like this floating around, it makes you realize we still don't know what sleep is. But there is a lot we do know about sleep.

One thing that is well studied are the Sleep phases humans go through as we age. When you are a new born you sleep all the time except when eating. By the end of the first year of life you are sleeping less but still sleep in definable phases. When you move into childhood you fall asleep early and sleep till dawn. Puberty though changes everything. You don't want to sleep until 3 or 4 in the morning and then sleep all day. Better known as the party years. As you move into young adulthood your sleep cycle shifts back to falling asleep around 8 to 9 PM and sleeping until sunrise. As we approach our later years we want to fall asleep in the late afternoon but then wake up at midnight or so and cant go back to sleep. Throughout all the phases though 6 to 8 hours of sleep seems to to be the biologic necessity.

What are the phases and stages of sleep?

Sleep is divided into two phases:

  1. Phase 1 Non-REM sleep

    1. Non-REM sleep is further sub-divided into three stages,

      1. Stage 1

      2. Stage 2

      3. Stage 3

  2. Phase 2 REM (Rapid Eye Movement) sleep.

    1. Rem Sleep

      1. Stage 4

Sequence of Sleep Stages

It's important to realize that sleep does not progress through the four stages in perfect sequence.

When you have a full night of uninterrupted sleep, the stages progress as follows:

  1. Sleep begins with NREM stage 1 sleep.

  2. NREM stage 1 progresses into NREM stage 2.

  3. NREM stage 2 is followed by NREM stage 3.

  4. NREM stage 2 is then repeated.

  5. Finally, you are in REM sleep.

Once REM sleep is over, the body usually returns to NREM stage 2 before beginning the cycle all over again. Time spent in each stage changes throughout the night as the cycle repeats (about four to five times total).(3)

Sleep architecture refers to the exact cycles and stages a person experiences in a night. A sleep specialist may show you this information on what's known as a hypnogram—a graph produced by an EEG.

What's Next?

So, now we have the basics of sleep. In part three (3) we will discuss the health benefits of sleep.


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