Study Objective: To determine whether exogenous melatonin improves day sleep or night alertness in emergency physicians working night shifts. Methods: In a double-blind, placebo-controlled crossover trial, emergency physicians were given 10 mg sublingual melatonin or placebo each morning during one string of nights and the other substance during another string of nights of equal duration. During day-sleep periods, subjective sleep data were recorded. During night shifts, alertness was assessed with the use of the Stanford Sleepiness Scale. Key outcome comparisons were visual analog scale scores for gestalt night alertness and for gestalt day sleep for the entire string of nights. Results: We analyzed data from 18 subjects. Melatonin improved gestalt day sleep (P=.3) and gestalt night alertness (P=.03) but in neither case was the improvement statistically significant. Of 13 secondary comparisons, 9 showed a benefit of melatonin over placebo; none showed a benefit of placebo over melatonin. Conclusion: Exogenous melatonin may be of modest benefit to emergency physicians working night shifts. [Jorgensen KM, Witting MD: Does exogenous melatonin improve day sleep or night awareness in emergency physicians working night shifts? Ann Emerg Med June 1998;31:699-704.]