This was a realistic military-type exercise assessing unexpected, abrupt early-morning awakening effects on immediate ‘executive function’ and the ability to comprehend and deal with a sudden emergency under a changing situation. Twenty (average age 21 years) healthy, highly motivated junior officer reservists were assigned randomly to two equal, independent groups, unforewarned as to what would happen. The experimental group was woken abruptly at 03 : 00 h (<3 h sleep) and confronted immediately with a ‘paper exercise’ of an enemy attack, requiring a feasible plan of engagement with minimal loss of resources, to be completed within 15 min. A control group slept until 07 : 30 h; they were then presented with the identical emergency 1 h later. Participants worked individually, under time pressure, receiving written information, map and other details, all containing relevant, irrelevant and misleading information. Halfway through, they were given (unexpectedly) a critical update necessitating a change of tactics. Performance was scored blind by instructors, under five categories. Eight of the experimental group versus three controls failed overall, with significant group differences on three specific categories relying on flexible decision-making: ‘identification of available cover’, ‘use of available assets’ and ‘extraction of relevant from irrelevant information’. Other, logical and highly trained skills were unimpaired. Ours was a ‘worst case scenario’, combining short sleep, circadian ‘trough’ and sleep inertia, all of which differentiated the two groups, unlike typical laboratory studies. Nevertheless, it was relevant to real-life situations involving highly motivated, trained individuals making critical innovative decisions in the early morning versus the normal waking day.