NREM sleep contributes to physical rest. Within NREM sleep, three stages are usually recognized: Stage 1,2, and 3. The sleep stages are primarily based on EEG criteria. The following is a description of each stage:
This is a transitional stage between waking and sleeping, usually lasting 5 to 10 minutes. Breathing becomes slow and regular, the heart rate decreases and the eyes exhibit slow rolling movements. On the polysomnogram, this stage is identified by a relatively low voltage, mixed frequency EEG.
This is a deeper stage of sleep where fragmented thoughts and images pass through the mind. Eye movements usually disappear, muscles relax, and there is very little body movement. On the polysomnogram, this stage is defined by the presence of sleep spindles and/or K complexes against the even background of the theta waves. This stage of sleep accounts for roughly 50% of adults' sleep time.
This third stage involves a further deepening of sleep with additional slowing of heart and breathing rates. The delta waves constitute ≥ 20 of the EEG in stage 3.In Stage 3, arousal is the most difficult. Usually for 30-40 minutes, there will be no change in the brain waves, no gross movements of the trunk of limbs, and it will be very difficult to awaken the subject. This sleep stage is often called deep sleep. This stage consists only of delta waves.