Disrupted circadian timing has emerged as a serious health and safety issue, yet there are no objective means of easily assessing circadian timing or alignment without performing a highly controlled, multiple-day in-patient study. Altered circadian timing can cause both sleep loss and sleepiness, cause performance errors such as motor vehicle accidents, and impair memory/learning. We are, therefore, using real-time breath metabolomics technology called select ion flow tube mass spectrometry (SIFT-MS) to explore whether the molecular constituents of exhaled breath can be used to determine circadian time of the individual. The ultimate goal of this research to enable determination of the circadian status from a single breath at the point-of-care or point-of-need.