Sleep 101

• Sleep is a physical and mental resting state in which a person becomes relatively inactive and unaware of the environment.

• Normal sleep is characterized by a decrease in body temperature, blood pressure, breathing rate and most other body functions but the brain remains active.

• Wakefulness is a state of awareness in which the individual is conscious of his/her surrounding environment and is able to react with it.

Sleep Cycle

Sleep is characterized by two distinct states, non-REM sleep and REM sleep that alternate in 90 to 110-minute cycles. A normal sleep pattern has 4-5 cycles throughout the night. Non-REM sleep consists of four stages that range from light dozing to deep sleep. Approximately 75% of night’s sleep is spent in non-REM sleep.

Stage 1 Sleep

is a transition period from wakefulness to sleep. It is characterized by slow rolling eye movements, otherwise known as the “dozing” stage. Approximately 5% of the the total night's sleep should be spent in Stage 1.

Stage 2 Sleep

is characterized by a lack of eye movements. Brain waves become larger. Defined on PSG with sleep spindles and K-complexes. Approximately 45% of the total night's sleep time is spent in Stage 2 of non-REM sleep. This stage is often considered the official onset of consolidated sleep. Eye movements stop and brain waves become larger. Stage 2 Sleep is graphically defined with two distinct brain wave forms called sleep spindles and K-complexes. A sleep spindle is a pattern of EEG waves, consisting of a burst of 11 to 15 hertz waves that last for .5 to 1.5 seconds. A K-complex is high voltage EEG activity that consists of a sharp downward component followed by a slower upward component and lasts more than .5 second.

Stage 3 and Stage 4 or Delta Sleep

Stage 2 sleep evolves into “Delta” sleep or “slow wave” sleep in approximately 10-20 minutes and may last 15-30 minutes. It is called “slow wave” sleep because brain activity slows down dramatically from the “theta” rhythm of stage 2 to a much slower rhythm of 1 to 2 cycles per second called “delta” and the height or amplitude of the waves increases dramatically. In most adults these two stages are completed within the first two 90 minute sleep cycles or within the first three hours of sleep. Contrary to popular belief, it is delta sleep that a sleep deprived person’s brain craves the first and foremost. In children unwakeable or “dead asleep” during most of the night.

Stage 5 Sleep

REM (Rapid Eye Movement Sleep) is a very active stage of sleep. Most dreaming takes place during REM sleep which is why you lose muscle tone in REM (so you don’t act out your dreams). REM sleep composes about 20 – 25% of the sleep cycle in normal adults. REM sleep is characterized by:

Eyelid Fluttering
Rapid Eye Movement
Muscle Paralysis
Irregular Breathing
Decrease in Body Temperature
Changes in Heart Rate & Blood Pressure

   
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