Aims and objectives: Sleep deprivation in the intensive care unit (ICU) has been linked to numerous complications. Light levels might impact the sleep of patients in the ICU. The aim of the study was to measure light levels during sleep-protected time in the ICU and to assess the impact of light intensity on sleep quantity/quality. Materials and methods: This prospective, observational study was conducted in a 10-bed, mixed surgical/medical ICU. For measuring light levels, a commercially available smartphone application was used. The measurements were performed between 23:30 and 06:15 hours at 15-minute intervals. To assess sleep quantity, we used Patient's Sleep Observation Behavioral Tool and to assess sleep quality, we used Richards-Campbell Sleep Scale. Results: The median number of time points at which patients were asleep was 20 (interquartile range, IQR 14–23) out of 25 (5 hours). The median self-reported quality of sleep (overall score) was 49 (IQR 28–71). The median values for individual questions are: question 1 (sleep depth)—54.0 (IQR 37–78), question 2 (sleep latency)—40.5 (IQR 6–90), question 3 (awakenings)—52.5 (IQR 28–76), question 4 (returning to sleep)—25.5 (IQR 11–78), and question 5 (sleep quality)—67.5 (IQR 5–76). No correlation was found between self-reported sleep quality and time spent asleep (p = 0.36). There was no correlation between average light levels during sleep-protected time and sleep quantity (p = 0.42)/sleep quality (p = 0.13). There was a correlation between average (13 ± 5 lux) light levels before sleep-protected time and sleep quality (p = 0.008). Conclusion: Mean light levels of 11 ± 9 lux during sleep-protected time have no negative impact on quantity and quality of sleep in intensive care unit patients. Light levels up to 18 lux directly before falling asleep improve patients’ self-reported quality of sleep in the ICU. Clinical significance: Finding safe levels of light intensity during sleep-protected time in ICU.