Odds Ratio Product as a Biological Marker of Phenotypes of Insomnia.

The study investigated differences in objective markers of sleep depth and identified phenotypes of insomnia. Participants were screened with the Insomnia-Severity-Index and clinical interviews and assigned to control (n = 50) or insomnia (n = 69) groups. They completed three nights of in-laboratory overnight polysomnography. We measured the Odds Ratio Product (ORP), a continuous measure of sleep depth (0 = deep sleep, 2.5 = full wakefulness) and calculated: (a) ORP in stages Wake, NREM, REM, (b) percentage of TRT in deep sleep (ORP < 0.5) and full-wakefulness (ORP > 2.25), (c) number/hour of sleep of transient increases in ORP to wake levels (Wake Intrusion Index [WII]), (d) gamma power, (e) frequency of alpha intrusions, (f) speed of return to deep sleep after arousals (ORP-9). We used Latent Class Analysis to differentiate two insomnia groups with 'Objectively Normal' and 'Objectively Poor' metrics. The Objectively Poor group had higher ORPwake, ORPNREM, ORPREM, %TRT > 2.25, gamma power, alpha intrusion, WII and ORP-9 than good sleeper (GSC) and the Objectively Normal group, illustrating evidence of hyperarousal, while the Objectively Normal group was comparable to GSC. The Objectively Poor group had higher %awake and lower TST. Both insomnia groups reported worse sleep and underestimated TST relative to GSC, despite similar objective sleep metrics in the Objectively Normal group. Using novel objective sleep metrics, we identified a subgroup of insomnia with abnormalities consistent with hyperarousal and another with no difference from GSC. Future research should test if these groups benefit from different treatment pathways and thus improve outcomes and time to determine appropriate treatments.