Background: Neurocritical care requires rapid and accurate monitoring to prevent secondary brain injuries in patients with neurological diseases. Automatic pupillometry is a tool for assessing pupillary response (Neurological Pupil index). The role of nurses is central in monitoring and caring for the patient's neurological status to minimize secondary brain injuries and suffering.  Currently, it is unclear if ambient light can influence the Neurological Pupil index. Objective: The aim was to evaluate if the Neurological Pupil index is affected of ambient light during automatic pupillometry in unconscious or sedated intensive care patients. Method: A crossover design where pupillometry measurements were performed in bright and dark rooms at six time points on 20 intensive care patients. The results were analyzed using the Wilcoxon signed-rank test and presented with descriptive statistics. Results: Significant differences in the Neurological Pupil index were observed between dark and bright conditions. The majority of participants showed higher NPi in dark environments compared to light. There was individual variation in NPi measurements, with some participants showing larger differences than others. Conclusion: NPi is higher in dark environments compared to bright ones, and there are also individual differences. The results can guide clinical practice to improve the critical care nurse's assessment of neurological status in patients. Consistent lighting conditions during neurologic assessments of patients may potentially enhance the assessment of pupillary reactions and potentially enhanced nursing care for the patient.