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Objectives: The objectives of this study were primarily to assess the efficacy and safety of SM-1 in a circadian challenge model of transient insomnia and secondarily, to assess the contribution of diphenhydramine to the combination., Methods: Randomized, double-blind, placebo-controlled three-way cross-over study with a 5-hr phase advance. Subjects were 39 healthy adults reporting a history of transient insomnia. All treatments (SM-1, SM-1 without diphenhydramine, or placebo) were administered to all subjects in a randomly assigned sequence, with at least 1 week between treatments. The primary endpoint was total sleep time (TST) determined by polysomnography. Secondary endpoints included wakefulness after sleep onset (WASO), latency to persistent sleep, number of awakenings (NAW), subjective TST (sTST) and sleep latency (sSL), TST, and NAW by quarters of the night and sleep quality. Safety endpoints included adverse events, Karolinska Sleepiness Scale digit symbol substitution test, and subject-reported alertness level., Results: SM-1 provided an increase of 126.7 min in TST over placebo (p < .001). WASO, sTST, sleep quality, and sSL also showed significant improvement. Diphenhydramine demonstrated a significant (p = .014) contribution of 43.7 min to TST. SM-1 was well-tolerated with type and frequency of adverse events comparable with placebo, and no residual sleepiness upon awakening after 8 hr., Conclusions: SM-1 provided a robust and statistically significant increase in TST compared with placebo in a circadian model of transient insomnia, without evidence of next-day impairment. Diphenhydramine contributed to the effect., (© 2019 John Wiley & Sons, Ltd.)