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Sleep Apnea

Sleep phases

From three to five 90-minute sleep cycles can occur at night; each is composed of five distinctive phases. The three first stages are characterized by non-rapid eye movement sleep and the fourth stage corresponds to paradoxical sleep or REM (rapid eye movement), which is the period during which we dream.


Stage 1 – Drowsiness

Drowsiness stage (transition between awakening and sleep)

The normal drowsiness time is between 10 to 30 minutes.

Reduction of heart rate, blood pressure, breathing rhythm and body temperature but remains regular for the balance of the night.

Muscle tone is slightly reduced, but is always present.

Eye movement is slow.


Stage 2 – Light sleep

Takes up about 50% of total sleeping time in a night.
During this stage, we are very sensitive to exterior stimulus and muscle tone remains reduced as in the first stage.

The heart rate, blood pressure and breathing rhythm are steady, at idle.

We observe little or no eye movements.


Stage 3– Slow and deep sleep

Slow and deep sleep corresponds to 20 to 25% of total sleeping time.
Heart rate, blood pressure, breathing rhythm and body temperature remain steady.

At stage 3, we do not observe eye movement but very discrete muscular activity. We also observe night terrors and somnambulism.

It is at this sleeping stage that secretion of several critical hormones take place:
  • growth hormones (important in children’s sleep);
  • appetite control hormones such as leptin;
  • immune system hormones (important for recovery);
  • sex hormones;
  • cortisol.


Stage 4 – Paradoxical sleep or REM (Rapid Eye Movement)

Sleep stage during which we dream.
Cerebral activity is very rapid and rapid eye movement (REM for Rapid Eye Movement) is observed.

Respiration and cardiac rates are irregular

Blood pressure is variable

Muscular tone is absent

REM stage is a very important phase for learning and memory operation and retention.