Two 9-d tactical defensive exercises were carried out. The first assessed and compared the performance of three platoons of infantry scheduled for either 0, 1.5, or 3 h of sleep in every 24 h, and the second determined whether soldiers are likely to remain militarily effective during a period of partial sleep loss following a period with no scheduled sleep at all. To this end, 10 infantry soldiers were scheduled for 4 h of sleep in every 24 for a 6-d period following a 3.75-d period without any scheduled sleep. Performance, physical fitness, and mood were assessed throughout both exercises. Results indicated that the effects of sleep loss are psychological rather than physiological; soldiers are likely to be militarily ineffective after 48-72 h without sleep; and a small amount of recovery sleep relative to the amount lost has very beneficial effects.