Abstract <p>The oil and gas industry is characterized by performing professional activities in extreme conditions (intensive work, difficult climatic conditions, etc.). Minimizing the negative impact of those factors that can be influenced by the employer is critical for the industry. Disturbances in the sleep–wake cycle are one of the key risk factors for the development of fatigue and, as a consequence, injuries and occupational diseases in any industry, especially in risk groups. The purpose of this study was to identify deviations in the sleep–wake cycle in individuals working in the oil and gas industry and having chronic diseases. It was shown that the duration of night sleep in 16.6% of oil and gas workers participating in the study was less than 6 h. Night sleep was characterized by an extension of falling asleep (more than 30 min) in 5.3% of the subjects, the duration of night awakenings of more than 1 h was noted in 6.8% of workers. According to the Berlin questionnaire, workers (14.6%) were identified who had a high probability of having obstructive sleep apnea. Sleep inertia lasting more than 2 h was observed in 8.8% of workers. Observed daytime sleepiness interfered with work in 14.0% of cases. Workers with chronic diseases compared to apparently healthy individuals had: a 3.5-fold increased risk of sleep apnea (35.0 versus 10.0%), worse sleep quality, increased time to fall asleep, a greater number of night awakenings, and pronounced daytime fatigue and sleepiness (all <i>p</i> ≤ 0.05). We assume that workers registered with a dispensary can be a group for the implementation of additional monitoring measures for the quality of both sleep and the wakefulness process. Implementation of complex measures (screening for deviations in the sleep–wake cycle, educational programs, improvement of working and rest conditions, promotion of a healthy lifestyle) will improve the overall performance of personnel, reduce the level of fatigue, and improve the health of workers in general.</p>