In a single-blind study six male patients (mean age 39.5 years) with moderate insomnia were treated with placebo for three nights, 100 mg indole-3-pyruvic acid (IPA) for three nights, 200 mg IPA for three nights, 100 mg IPA for two nights and placebo for two nights. Polygraphic recordings were made and total sleep time, sleep efficiency, sleep latency, slow wave sleep latency, rapid eye movement (REM) sleep latency, number of arousals (greater than 1 min), percentage and duration of wakefulness after sleep onset, percentage and duration of wakefulness after sleep onset, percentage and duration of sleep stages 1, 2, 3, 4 and REM were recorded. At the end of 13 days, total sleep time, duration of stage 2 sleep and total non-REM were significantly increased when compared with baseline. Total sleep time and duration of stage 2 and total non-REM sleep on completion were significantly decreased when compared with after 200 mg IPA (night 9). Results suggest an action of IPA on human sleep similar to that of exogenous melatonin and L-tryptophan, thus confirming that IPA could be used to increase serotonin and melatonin turnover.