For this paper, psychophysiological relationships between stress and sleep are considered with reference to clinical psychiatric problems. Answers to the first question--how does sleep affect the tolerance of stress in the waking state?--are provided by sleep deprivation experiments. The latter show that total sleep deprivation has a great influence on stress. However gradual partial sleep deprivation to a sleep time of 5 hours seems well tolerated by most individuals. In some special cases we found subjects well adapted to the workaday world who could live with briefer sleeping periods. Because of experimental problems, the second question--how does stress affect sleep?--is more difficult to answer. Anxiety is one of the most important factors deteriorating sleep under conditions of stress. This reduced sleep is in turn stressful for waking behavior. Thus, the relationship between stress and sleep, two factors separated for scientific investigation, is essential and especially important in the problem of insomnia.