Does melatonin have an effect on cognitive performance?

Exogeneous melatonin causes phase shifts of circadian rhythms, reduces core body temperature, and induces drowsiness, but dispute still surrounds the hormone's effect on cognitive performance. Using a N = 1 double-blind alternating treatments design, daytime performance on four cognitive tests (logical reasoning, serial add/subtract, visual-spatial, and four-choice reaction time tasks) was measured after PO doses of placebo and 1.6 mg melatonin. Serum melatonin and core body temperature were also recorded. Performance was measured during time windows corresponding to the serum melatonin peak and the temperature trough respectively, following melatonin administration. The data showed that performance speed and accuracy measures were not affected during the serum melatonin peak, but that reaction times on all cognitive tasks increased substantially during the temperature trough. These results suggest that circulating melatonin has no direct and immediate effect on cortical information processing, and that the reduction in cognitive processing speed during body temperature trough occurs as a consequence of the hormone's hypothermic properties, causing a slowing down of cerebral processing speed.