OBJECTIVE: Because of around-the-clock activities, environmental noise and light are among the many causes of sleep disturbance in an intensive care unit (ICU). The implementation of guidelines may potentially change behavior rules and improve sleep quality. DESIGN: A prospective interventional study, observing the effects of simple nighttime guidelines on light and noise levels in an ICU. SETTING: A modern surgical ICU, subdivided into six identical three-bed rooms. PATIENTS: Critically ill adult patients. INTERVENTION: Between two observation periods, five guidelines were implemented to decrease both light and noise during the night shift in the patient's room. MEASUREMENT: Light levels and noise levels were obtained using a luxmeter and a sound level meter [A-weighted decibels (dB) scale] and were monitored continuously from 11 pm to 5 am both before (period P1) and after (period P2) the implementation of guidelines. MAIN RESULTS: Similar patient's gravity and nursing workload scores were observed between P1 and P2. A low mean (<5 Lux) and maximal light level were measured during both P1 and P2. The implementation of guidelines lowered mean light disturbance intensity with a greater variability of light during P2. All noise levels were high and corresponded more to a quiet office for noise level equivalents and to a busy restaurant for peak noise levels during both P1 and P2. Guidelines decreased the noise level equivalent (P1, 51.3 dB; P2, 48.3 dB), peak noise level (P1, 74.9 dB; P2, 70.8 dB), and the number of acoustic identified alarms (P1, 22.1 dB; P2, 15.8 dB) during P2. CONCLUSION: The night light levels were low during both periods, and lowering the light levels induced a greater variation of light, which may impair sleep quality. All measured noise levels were high during both periods, which could contribute to sleep disturbance, and the implementation of guidelines significantly lowers some important noise levels. The background noise level was unchanged.