The aim of the study is to evaluate whether occupational exposure to night work could cause alterations in the levels of plasmatic cortisol. The interest toward this argument arises form several studies in scientific literature referring the presence of an alteration in the synthesis and release of cortisol in workers exposed to night work. We studied a population of workers employed in night security service and monitoring service of alarm systems in different museums compared to a control group not performing shift-work and/or night work. The exposed and control subjects were compared by age, length of service, smoking habit (n. cigarettes per day), habitual consumption of alcoholic drinks (n. glass of wine/beer per day). We evaluated the levels of plasmatic cortisol on 50 workers exposed to night work, all males of whom 30 smokers and 20 non-smokers and on 50 controls of whom 30 smokers and 20 non-smokers.