Sleep deprivation is an unpleasant burden of young hospital doctors during their medical training. It may disrupt the balance between coping strategies available to them and the professional demands encountered. Impaired medical care offered by sleep-deprived juniors may be a consequence. Valid research work on this subject is rare and surprisingly contradictory. Therefore, we evaluated the task-specific cognitive status and emotional condition of 40 young hospital doctors (27 men and 13 women, 29.9 +/- 2.9 years of age) at the University of Tuebingen, all of whom were in the beginning of their academic career. Subjects were tested twice acting as their own control, once at 8.00 am after a night off duty (OD) (at least 6 hours of uninterrupted sleep), and once at a similar time after a night on call (OC) being in the hospital for 24 hours. Standardized and reliable psychometric tests thought to represent daily routine medical function were performed. On-call activities were recorded by means of a sleep diary, whereas a questionnaire interrogated aspects of private and professional life. Neuropsychological function deteriorated significantly: number connection test (per cent of norms +/- SD, 103.2 +/- 9.8 OC vs 107.8 +/- 10.5 OD, F = 27.7, P < 0.001), things-to-do list (correct items +/- SD, 6.7 +/- 1.2 OC vs 7.4 +/- 1.5 OD, F = 12.7, P < 0.01), Vienna reaction timer (per cent of norms +/- SD, 95.6 +/- 9.0 OC vs 97.7 +/- 10.4 OD, F = 4.8, P < 0.05), Stroop test (T-values +/- SD, 59.7 +/- 6.3 OC vs 64.6 +/- 7.1 OD, F = 37.1, P < 0.001), ECG test (correct responses +/- SD, 38.3 +/- 7.3 OC vs 43.4 +/- 6.5 OD, F = 45.2, P < 0.001) and status of mood (T-value +/- SD, 60.3 +/- 9.0 OC vs 54.0 +/- 6.6 OD, F = 19.6, P < 0.001). Cognitive function and mood status of young hospital doctors after a night on call decrease considerably. In view of the special vulnerability of medical trainees to occupational stress all efforts are warranted to reduce sleep deprivation in the medical profession.