The Sleep Cycle

CHAPTER 4

  

The Sleep Cycle

PART 1

Sleep Cycles: REM and NREM

Sleep can be divided into two types: rapid eye movement (REM) sleep, and non-rapid eye movement (NREM) sleep. Most dreaming occurs during REM sleep.

According to the most recent standards of the American Academy of Sleep Medicine, NREM sleep can be further divided into three stages: N1, N2, N3. Normally, your sleep proceeds in cycles of REM and NREM sleep, in this order: N1 - N2 - N3 - N2 - REM. This represents one cycle. When one cycle is completed, another starts, beginning again with stage N1. A complete sleep cycle takes about 90-110 minutes. A night’s sleep may include four to six sleep cycles. READ MORE

NREM Sleep

  • Stage N1 (light sleep). In this stage, you drift in and out of sleep. You can easily be awakened from stage 1 sleep, and if you are, you may remember fragmented visual images.

  • Stage N2. Eye movements and brain waves slow down, with occasional sleep spindles (short bursts of brain activity). Consciousness of the external environment disappears. This stage occupies about half of total sleep time for most adults.

  • Stage N3 (deep or delta sleep). Very slow brain waves called delta waves appear, interspersed with smaller, faster waves. This is the stage in which night terrors and sleepwalking occur.


REM Sleep
REM sleep accounts for about 10-25% of total sleep time. Typically, REM sleep begins about 70-90 minutes after you fall asleep. In REM sleep, you have rapid, jerky eye movements and rapid low-voltage EEG. Your heart rate increases and your blood pressure rises. Limb muscle atonia—temporary limb muscle paralysis—sets in.

In the first sleep cycles of the night, it’s typical to have short REM periods and long periods of deep NREM. As the night goes on, periods of deep sleep decrease and REM sleep periods increase in length. By the morning, almost all your sleep time is spent in stages N1, N2, and REM. LESS
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PART 2

Physiological Changes During Sleep

Your body undergoes numerous physiological changes during sleep, which differ from wakefulness during REM and NREM periods as follows:

Physiological Process During NREM During REM
Brain activity Decreases Increases in motor and sensory areas
Heart rate Slows Increases and varies compared with NREM
Blood pressure Decreases Increases (up to 30 percent)
Blood flow to brain Doesn’t change in most regions Increases by 50-200%
Respiration Decreases Increases and varies from NREM
Airway resistance Increases Increases and varies
Body temperature Set point lowers Not regulated
Sexual arousal Occurs infrequently Increases from NREM


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Other physiological changes:

  • Endocrine (hormonal) system. Many hormones are secreted into your blood when you sleep, including growth hormone, linked to repair processes that take place during sleep; follicle stimulating hormone and luteinizing hormone, necessary for maturational and reproductive processes; and prolactin, important in maintaining immune system function.

  • Renal (kidney) system. Your kidneys’ filtration and excretion of electrolytes are reduced during sleep, making your urine more concentrated.

  • Digestive system. Gastric acid secretion is reduced when you sleep.

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