Objectives: Ayahuasca is a traditional South American psychoactive beverage and the central sacrament of Brazilian- based religious groups, with followers in Europe and the United States. The tea contains the psychedelic indole N, N-dimethyltryptamine (DMT) and β-carboline alkaloids with monoamine oxidase-inhibiting properties that render DMT orally active. DMT interacts with serotonergic neurotransmission acting as a partial agonist at 5-HT1A and 5-HT2A/2C receptor sites. Given the role played by serotonin in the regulation of the sleep/wake cycle, we investigated the effects of daytime ayahuasca consumption in sleep parameters. Measurements and results: Subjective sleep quality, poly- somnography (PSG), and spectral analysis were assessed in a group of 22 healthy male volunteers after the administra- tion of a placebo, an ayahuasca dose equivalent to 1 mg DMT kg−1 body weight, and 20 mg d-amphetamine, a proaminergic drug, as a positive control. Results show that ayahuasca did not induce any subjectively perceived deterioration of sleep quality or PSG-measured disruptions of sleep initiation or maintenance, in contrast with d- amphetamine, which delayed sleep initiation, disrupted sleep maintenance, induced a predominance of ‘light’ vs ‘deep’ sleep and significantly impaired subjective sleep quality. PSG analysis also showed that similarly to d- amphetamine, ayahuasca inhibits rapid eye movement (REM) sleep, decreasing its duration, both in absolute values and as a percentage of total sleep time, and shows a trend increase in its onset latency. Spectral analysis showed that d-amphetamine and ayahuasca increased power in the high frequency range, mainly during stage 2. Remarkably, whereas slow-wave sleep (SWS) power in the first night cycle, an indicator of sleep pressure, was decreased by d-amphetamine, ayahuasca enhanced power in this frequency band. Conclusions: Results show that daytime serotonergic psychedelic drug administration leads to measurable changes in PSG and sleep power spectrum and suggest an interaction between these drugs and brain circuits modulating REM and SWS.