Sleep Cycle - The progression through an orderly succession of sleep states and stages. In a healthy adult, the first cycle is always initiated by going from wakefulness to non-REM sleep. The first REM period follows the first period of non-REM sleep to complete the first sleep cycle. The two sleep states continue to alternate throughout the night with an average cycle period of about 90 minutes. A full night of normal human sleep will usually consist of 4-6 non-REM/REM sleep cycles.

Sleep Stages - Distinctive stages of sleep as demonstrated by EEG patterns. See Stages 1-4; REM Sleep.

Sleep Stage 1 (NREM Stage 1) - A stage of NREM sleep that ensues directly from the awake state. Its criteria consist of a low-voltage EEG tracing with well defined alpha activity and theta frequencies in the 3 to 7 cps range, occasional vertex spikes, and slow rolling eye movements (SEMs), and includes the absence of sleep spindles, K complexes, and REMS. Stage 1 normally represents 4-5% of total sleep.

Sleep Stage 2 (NREM Stage 2) - A stage of NREM sleep characterized by the advent of sleep spindles and K complexes against a relatively low-voltage, mixed frequency EEG background. High voltage delta waves may comprise up to 20% of stage 2 epochs. Stage 2 usually accounts for 45-55% of total sleep time.

Sleep Stage 3 (NREM Stage 3) - A stage of NREM sleep defined by at least 20% and not more than 50% of the period consisting of EEG waves of 2 cps or slower, with amplitudes of more than 75 mV (high-amplitude delta waves). It constitutes "deep" NREM sleep, or, so-called slow wave sleep (SWS), and is often combined with stage 4 NREM sleep because of the lack of documented physiological differences between the two. It normally appears usually only in the first third of the sleep episode, and usually compromises 4-6% of total sleep time.

Sleep Stage 4 (NREM Stage 4) - All statements concerning NREM sleep stage 3 apply to stage 4 except that high-voltage, slow EEG waves cover 50% or more of the record. NREM sleep stage 4 usually represents 12-15% of total sleep time. Sleepwalking, sleep terrors, and sleep-related enuresis episodes generally start in stage 4 or during arousals from this stage.

REM Sleep (Rapid Eye Movement Sleep) - The sleep stage in which vivid dreaming occurs; identified by the occurrence of rapid eye movements under closed eyelids, motor atonia and low voltage EEG patterns. Also associated with bursts of muscular twitching, irregular breathing, irregular heart rate, and increased autonomic activity.

Ted - you're the best! - Thanks

Hi Ted,
You mentioned that REM stage is where "vivid dreaming" occurs. Does ANY dreaming occures in any other stages? Some of these medications (eg. SSRIs) are meant to lift depression/insomnia. Do they adversely affect sleep at the same time? It seems like the chicken and egg issue !!! They are supposed to make things better while they make the sleep worst.
Thanks.