Effects of Sleep Deprivation on Exhaled Nitric Oxide Concentrations in Medical Residents Taking Night Call Background: Exhaled nitric oxide (eNO) is considered a biomarker for airway inflammation, and has been studied most extensively in asthma. It has also been implicated as a biomarker in other forms of stress, including psychological, although results have been variable. Sleep deprivation in medical residents on duty can be stressful. Objective: The purpose of this study was to determine whether sleep deprivation was linked to altered levels of eNO and, if so, whether eNO concentrations (FeNO) could be reliably used as a simple, non-invasive measure of inflammation in varying degrees of sleep deprivation. Our hypothesis was that FeNO levels are higher during sleep deprivation. Given that taking night call can be a stressful situation (leading to sleep disruption), we asked (1) what is the relationship between FeNO in physician trainees and sleep deprivation, and (2) can FeNO be viewed as a marker for inflammation in sleep deprivation during on-call duty?