Evaluation of sleep quality and sleep-disturbing factors in patients in an Intensive Care Unit (ICU). Over 8 months, 83 persons were examined as part of the recruitment of patients hospitalised in the ICU. The interviewer conducted interviews with the patients on the third day following their discharge from the ICU. Patients had to successfully complete the Short Portable Mental Status Questionnaire. The patients whose results reflected their normal mental status were administered the Richards–Campbell Sleep Questionnaire to evaluate the quality of sleep. A modified Freedman questionnaire was then administered. It was discovered that factors which disturb sleep most include the measurement of vital signs (3.28/10) and light (3.18/10) (1 = no disturbance, 10 = significant disturbance). It was also determined that the higher the VAS reported during the first day in the ICU, the higher the importance of pain in sleep disturbance in the ICU, Z = − 2.65; p < 0.05. The study results indicate that sleep disturbance in an ICU results from many factors. A comprehensive approach to a patient’s night rest is required. A reduction of sleep-disturbing factors could result in an improvement of the quality of sleep in an ICU. What remains to be done is to turn the attention of the medical staff working in an ICU to the importance of sleep and raise their awareness of the factors which may affect it.