Effect of a Light Intervention on Morning Sleep Inertia Symptoms

Introduction: Sleep inertia—sleepiness and performance impairment experienced upon waking—improves with light exposure following nocturnal awakenings both in the laboratory and real-world environments. However, light exposure at habitual waketime appears to be less effective. This study investigated the effects of light exposure on sleep inertia at habitual morning waketime in a real-world environment.

Methods: Twenty-nine healthy adults (16 female; mean age 27.1 ± 5.1 y), kept habitual sleep patterns at-home monitored by actigraphy for a week. Following experimental night 7 (monitored by electroencephalography, EEG), participants were woken at their habitual morning waketime via a phone call. Participants were randomized by sex and night-order to either wear light-emitting glasses with the light on (intervention) or off (control) during a test battery that included a 5-minute visual Psychomotor Vigilance Task (PVT; NASA PVT+ App), the Karolinska Sleepiness Scale (KSS), mood and alertness visual analog scales (VAS), and a 3-minute auditory Descending Subtraction Task (DST). Testing occurred at 2, 12, 22, and 32 minutes after waking. Testing sessions were monitored remotely via web-based cameras. Data were analyzed using mixed-effect models with fixed effects of test bout, condition, and their interaction, and a random effect of participant. Baseline performance, duration of ambient light exposure > 10 lux during the testing period (Actiwatch Spectrum PRO), total sleep duration from the prior night (EEG), sex, and randomization order were included as covariates.

Results: There was a significant main effect of condition for PVT speed with those in the light condition performing worse than the control condition (p = .049). There was a significant main effect of test bout for PVT lapses (p = .028), DST total responses (p = .023), DST correct responses (p = .001), VAS alert (p < .001), with impairment dissipating across time since waking. There were no other significant main or interaction effects (p > .05).

Conclusion: These findings align with previous studies suggesting that light exposure has limited effectiveness after waking in the morning. Worse performance on the PVT may be due to glare from the light glasses interfering with screen visibility. Further exploration of delivery methods for light exposure is needed.