Standards of polygraph examination exclude testing sleep-deprived people. Lack of sleep (and examinee exhaustion) is a factor that can contribute to false polygraph examination results (including false positives). Based on behavioral assessments (careful observation of examinee behaviour during a pre-test interview and subsequent stages of polygraph examination), the critical role of examiners is to recognise incidences of lack of sleep and, consequently, to postpone the moment of testing. Professional literature treats the lack of sleep as a temporal inability to be subjected to the test (Widacki, 2018, 434). However, such a decision is nowadays usually powered with experts’ intuition (their experience) rather than the results of empirical research. There is therefore a need for conducting studies like the one presented below. In practice, a sleep-deprived person could take a polygraph examination for two main reasons. First, examinees may deliberately deprive themselves of sleep to interfere with the results of tests. Secondly, such a deprivation may be connected with external circumstances of a particular examination. In a case involving a jewelry store robbery in Katowice, police officers wanted an expert to examine the building’s security staff. They wanted to have outcomes of screening tests on the day of the theft. The expert refused to perform the test because the security guards were tired after the night shift. In any event, a polygrapher should not yield to pressure and test sleep-deprived persons. The main goal of the research reported in the present article is to explore the influence of sleep deprivation of tested subjects on the accuracy of polygraph test results.