Sleep Basics

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.

The stages of sleep are determined with a polysomnogram (PSG). The PSG records vital signs and physiology during sleep including:


Electro-encephalogram (EEG)
Electro-myograph (EMG)
Electro-oculograph (EOG)
Respiratory airflow
Blood oxygen saturation
Pulse rate
Heart rate
Body position
Respiratory effort

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. The “dozing” stage.Approximately 5% of the the total night's sleep is 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.

In Stage 3 sleep, an individual becomes difficult to arouse. On the PSG, large and slow delta waves appear. Approximately 12% of the total night's sleep time is spent in Stage 3 of non-REM sleep.

Stage 4 Sleep is characterized by very deep sleep. When awakened, person feels groggy and disoriented.PSG shows delta waves almost exclusively. Approximately 13% of the total night's sleep time is spent in Stage 4 of non-REM sleep.

REM Sleep

Most dreaming takes place during REM sleep. REM sleep makes up 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 and blood pressure