Abstract
Depth of sleep within non-rapid-eye-movement (non-REM) stage-2 (N2-depth) varies from being nearly similar to stage-1 to nearly similar to stage-3. Differences in N2-depth are not captured by conventional indices of sleep depth (N1–N3 times, arousal/awakening index) when comparing sleep depth under different conditions. Magnitude of changes in N2-depth in the same individual under different experimental conditions, and the importance of measuring these changes are unknown.
We measured sleep depth using the odds-ratio-product (ORP), a continuous index of sleep depth (0 = deep sleep, 2.5 = full wakefulness) (Younes et al. (2015) ORP of sleep EEG as a continuous measure of sleep state. Sleep 38(4):641–54.). ORP in stage 2 (ORPN2) and in all non-REM (ORPNR) were measured, along with conventional indices (1) in the first and second halves of the night in 34 patients with no pathology, and (2) before and on continuous positive airway pressure (CPAP) in 75 patients with obstructive apnea.
In most (31/34) ‘no pathology’ patients, ORPN2 and ORPNR were unchanged (ΔORP −0.05 to 0.05; N = 15) or higher (0.1–0.63) in the second half. Changes in ORPN2 on CPAP were bidirectional and often large (5–95% confidence interval was −0.62 to 0.25). There was an excellent correlation between ΔOPRN2 and ΔORPNR in both groups (r > 0.90) emphasizing the importance of changes in N2-depth, when they occur, to changes in overall sleep depth. Changes in N1 time, N3 time and arousal/awakening index also correlated, but less strongly, with ΔORPNR (r = 0.76, −0.54, 0.69, respectively).
N2 sleep depth can change dramatically in the same individual under different conditions. Ignoring these changes may result in inaccurate assessment of changes in overall non-REM sleep quality.
•Intervention-related changes in N2 sleep depth are not captured by current indices.•ORP is a new index that measures instantaneous sleep depth on a continuous scale.•ORPN2 changes substantially with sleep time and following CPAP administration.•Changes in overall non-REM depth are largely determined by changes in N2-depth.•N2 depth needs to be considered in studies that evaluate changes in sleep depth.