Our previous study demonstrated that phlorotannin supplement had a sleep-promoting effect in rodents. In the present study, we investigated whether the phlorotannin supplement could improve sleep in subjects with self-reported sleep disturbances. In a randomized, double-blind, placebo-controlled trial, 24 subjects consumed either a placebo or phlorotannin supplement (500 mg/day) for 1 week, 30-60 min prior to bedtime. Sleep parameters were assessed at baseline and at 1 week with sleep questionnaires and polysomnography. At the end of the treatment period, the complete sets of sleep parameters from 20 subjects. Phlorotannin resulted in a significant increase in "Sleep duration" scores compared to the placebo (p = .044), although there were no significant differences on the total PSQI scores. Polysomnography revealed that wakefulness after sleep onset was significantly lower in the phlorotannin group compared to the placebo group (phlorotannin vs. placebo, -25.5 ± 30.5 vs. -1.7 ± 14.9; p = .045) as well as total wake time (phlorotannin vs. placebo, -0.9 ± 3.0 vs. -6.1 ± 6.8; p = .048). Additionally, the respiratory disturbance index during supine rapid eye movement sleep was significantly lower in the phlorotannin group (p = .035). There were no serious adverse effects in either group. Our data suggest that the phlorotannin supplement improved sleep maintenance (WHO ICTRP: KCT0001892).