Glossary of Sleep-related terms

Stage 1 (NREM Stage 1)

The lightest phase of NREM sleep that directly follows the awake state. It is a transition between wakefulness and sleep, and an individual is easily aroused from this stage. It is characterized by mixed frequency, low-voltage EEG patterns with bursts of alpha activity and theta frequencies in the 3 to 7 cps range, occasional vertex spikes. Another feature is slow, rolling eye movements (SEMs). There is an absence of sleep spindles, K complexes, and REMS. In adults, stage 1 normally represents 4-5% of total sleep. However, in some sleep disorders, such as sleep apnea, this percentage of stage 1 increases. A person with sleep apnea must wake up to breath, and stage 1 follows every arousal.1