Can slow eye movements occur in Non-Rapid Eye Movement (NREM) 2 sleep?

Slow Eye Movements in N2 Sleep

Yes, slow eye movements (SEMs) can occur during N2 sleep, though they are more characteristic of N1 (transitional) sleep and typically decrease as sleep deepens into N2 stage. 1

Characteristics of Eye Movements Across Sleep Stages

• N1 sleep is considered transitional sleep where slow eye movements are most prominent and frequent (0.46 ± 0.4 scratches/min in studies of atopic dermatitis patients), with arousal from this state being easiest 1
• In N2 sleep, slow eye movements can still occur but are less frequent than in N1, with arousal thresholds during N2 and REM sleep being similar 1
• N3 sleep (deep sleep) has characteristic slow waves on polysomnogram and has the highest arousal threshold, with minimal eye movements 1
• REM sleep is characterized by rapid eye movements, muscle atonia, and desynchronized EEG 2

Physical Properties of Slow Eye Movements

• SEMs during sleep onset and N1 sleep typically have moderate amplitude, slow velocity, and short duration 3
• As sleep progresses from N1 to N2, the physical properties of SEMs change, with SEMs in N2 generally having different amplitude and velocity characteristics than those in N1 3
• SEMs can be distinguished from rapid eye movements by their slower velocity and different waveform morphology 3

Sleep Stage Classification and Eye Movements

• According to the American Academy of Sleep Medicine (AASM) scoring manual, eye movements are one of the key physiological parameters used to differentiate between sleep stages 1
• The recommended EOG montage for detecting eye movements includes electrode placements that can identify both slow and rapid eye movements 1
• Slow eye movements are particularly important in identifying the transition from wakefulness to sleep (N1) but can persist into N2 sleep 1

Clinical Significance

• The presence and characteristics of slow eye movements can help in accurately staging sleep, which is important for diagnosing sleep disorders 1
• In certain conditions like atopic dermatitis, the frequency and pattern of eye movements during different sleep stages can be altered, affecting sleep quality 1
• Understanding the normal pattern of eye movements across sleep stages is essential for interpreting polysomnographic findings in both healthy individuals and those with sleep disorders 1

Stability of Sleep Stages and Eye Movements

• Sleep can be categorized as stable or unstable within each stage (NREMS vs. NREMUS), with different patterns of eye movements, breathing, and autonomic activity 4
• In stable N2 sleep (NREMS), eye movements are typically minimal compared to unstable N2 (NREMUS) where more variability may be observed 4
• The presence of slow eye movements in N2 may sometimes indicate a transitional state or partial arousal 1, 4

In summary, while slow eye movements are most characteristic of N1 (transitional) sleep, they can and do occur during N2 sleep, though typically with decreasing frequency as sleep deepens from N1 to N2 and then to N3.