Not all N3 sleep is equal: EEG-defined N3 sleep depth and health-related quality of life in the sleep heart health study.

OBJECTIVE: N3 sleep is commonly interpreted as restorative sleep, but whether the physiological depth of N3 sleep provides information beyond N3 duration is unclear. We examined the independent and joint associations of N3 duration and electroencephalography-defined N3 depth with daytime sleepiness and health-related quality of life. METHODS: We analyzed baseline data from 5469 Sleep Heart Health Study participants with N3 duration and odds ratio product during N3 sleep (ORPN3). Higher ORPN3 indicates shallower sleep within conventionally scored N3 epochs. Outcomes were the Epworth Sleepiness Scale (ESS), SF-36 physical component summary (PCS), and SF-36 mental component summary (MCS). Primary multivariable linear regression analyses examined standardized N3 duration and standardized ORPN3, supportive analyses examined four median-defined N3 quantity-depth phenotypes: long-deep, long-shallow, short-deep, and short-shallow. Secondary analyses assessed nonlinearity, excluded participants with insomnia, and related baseline sleep measures to repeated SHHS2 outcomes after adjustment for corresponding baseline scores. RESULTS: N3 duration and ORPN3 were weakly correlated (Spearman rho = -0.047). N3 duration showed no significant linear association with ESS, PCS, or MCS. In mutually adjusted models, each 1-SD higher ORPN3 was associated with lower PCS (β = -0.49; 95% CI, -0.76 to -0.22) and MCS (β = -0.71; 95% CI, -0.95 to -0.47), but not ESS. These associations persisted after excluding insomnia, with no evidence of nonlinearity for ORPN3. Longitudinally, higher baseline ORPN3 was associated with lower follow-up PCS (β = -0.41; 95% CI, -0.73 to -0.08), but not MCS or ESS. CONCLUSIONS: N3 duration and EEG-defined N3 depth represent distinct dimensions of deep sleep. Shallower N3 sleep was associated with poorer baseline health-related quality of life independent of N3 duration. Longitudinal evidence was limited to physical health-related quality of life.