Normal Sleep Architecture And Sleep Stages

Natural Insomnia Program

Insomnia Causes and Treatment

Get Instant Access

Sleep can be divided into two states: nonrapid eye movement (NREM) and rapid eye movement (REM) sleep. Each cycle of NREM and REM sleep lasts approximately 90 to 110 minutes, and about three to six cycles of NREM-REM sleep occur during a normal night of sleep (Fig. 1). Slow wave sleep (SWS), which will be discussed in more detail later, tends to dominate the first third of sleep, whereas REM sleep is most predominant in the last third of sleep. The first REM cycle is typically short; the final cycle of REM tends to be the longest. In terms of arousa-bility, the deepest sleep occurs in the first third of the night, corresponding to SWS (NREM stages III and IV); an individual awakened during these stages of sleep is typically groggy and confused. If awakened during lighter stages of NREM sleep (stages I and II), there is the possibility that the individual may not be aware that he or she had fallen asleep. However, if one were awakened during REM sleep, a person may experience residual sleep paralysis or persistence of dreams intruding into the waking state because paralysis of voluntary muscles and dreams are normal components of REM sleep. Typical behavioral events that are observed following sleep onset, irrespective of type of sleep, include transient amnesia for the occurrence of sleep, fragmentary images, hypnic jerks, and automatic behavior.

An individual's sleep can be recorded via an overnight polysomnogram (PSG). Some of the most commonly measured variables include electroencephalography (EEG), electrooculography (EOG), electromyography (EMG), electrocardiography, respiration, oxygen saturation, snoring, and body position (Fig. 2). To score the various stages of sleep, an epoch of sleep, or a 30-second

FIGURE 1 Sleep hypnogram demonstrating typical shifting sleep stages during a single night.

polygraphic tracing at a speed of 10 mm/second, is evaluated according to defined criteria set forth in the international classification of sleep disorders (ICSD) (1).

In human adults, NREM sleep occupies 75% to 80% of the total sleep time. It is comprised of stages I through IV. Stage I sleep accounts for about 3% to 8% of the total sleep time. It is scored when an alpha rhythm (8-12 Hz) is present in less than half of the epoch. Theta (4-7 Hz) and beta frequencies (> 13 Hz), slow rolling eye movements, and a slight decrease in electromyographic activity may be present in this stage of sleep. At the end of stage I, vertex waves lasting 50 to 200 msec can occur.

The largest portion of the total sleep time, about 45% to 55%, is spent in stage II sleep. Sleep spindles are present at this stage. These waveforms are generally 12 to

FIGURE 2 (A) Wakefulness: desynchronized EEG, and very active chin EMG activity. One random eye blink. (B) Stage II: Alpha rhythm EEG with K complex and reduced but still active chin EMG. (C) Slow wave (delta) sleep: slow delta activity EEG and still active chin EMG. (D) REM: desynchronized EEG and no chin EMG activity. Abbreviations: EEG, electroencephalography; EMG, electromyography; REM, rapid eye movement.

FIGURE 2 (A) Wakefulness: desynchronized EEG, and very active chin EMG activity. One random eye blink. (B) Stage II: Alpha rhythm EEG with K complex and reduced but still active chin EMG. (C) Slow wave (delta) sleep: slow delta activity EEG and still active chin EMG. (D) REM: desynchronized EEG and no chin EMG activity. Abbreviations: EEG, electroencephalography; EMG, electromyography; REM, rapid eye movement.

14 Hz, with duration of about 0.5 seconds and have a ''spindle-shaped'' appearance. K complexes are waveforms with a negative wave followed by a positive wave; both waves last for greater than 0.5 seconds. Less than 20% of the epoch contains delta waves (0.5-4 Hz) in this stage of sleep.

SWS includes both stage III and stage IV sleep. Stage III is identified by the presence of delta waves (0.5-4 Hz) in 20% to 50% of the epoch. About 3% to 8% of the total sleep time is accounted for by this stage.

Stage IV sleep is characterized by the presence of delta waves in greater than 50% of the epoch and accounts for about 10% to 15% of the total sleep time. Body movements are registered as artifacts in PSG recordings at the end of SWS. In stages II through IV, eye movements are not present, and muscle tone is decreased from that seen in either wakefulness or stage I (2).

About 20% to 25% of total sleep time is spent in REM sleep. The first cycle of REM sleep occurs approximately 60 to 90 minutes after the onset of NREM sleep. A characteristic finding in REM sleep is the ''saw tooth'' appearance of theta waves on electroencephalographic tracings. Within REM sleep, tonic and phasic stages occur. The hallmarks of tonic REM sleep include hypotonia or atonia of major muscle groups, desynchronization of the EEG, and depression of monosynaptic and polysynaptic reflexes. Phasic REM sleep is characterized by tongue movements, REMs in all directions, phasic swings in blood pressure and heart rate, irregular respiration, spontaneous middle ear muscle activity, and myoclonic twitching of chin and limb muscles (3-6).

Was this article helpful?

0 0
Natural Cures For Insomnia

Natural Cures For Insomnia

How Would You Love To Finally Get A Decent Night's Sleep? Do you toss and turn all night long without getting much of a wink of sleep? Are you tired of counting sheep for hours without falling asleep? Wouldn't you love to be able to fall asleep simply, easily and naturally, without pills, potions or harmful medicine?

Get My Free Ebook


Post a comment