Previous research conducted by DRDC Toronto shows that human circadian rhythms can be manipulated using appropriately-timed treatments of light and/or supplementary melatonin. The seasonal extremes of photoperiod in the high Arctic places particular strain on the human circadian system, which may lead to Seasonal Affective Disorder (SAD) in the winter months and difficulty obtaining sufficient sleep in the summer months. The goal of the work reported here was to study the circadian rhythms of personnel deployed to the high Arctic by tracking the melatonin produced by the body and identifying the timing of daily onset of melatonin production (DLMO, or Dim Light Melatonin Onset). This will permit follow-up work and recommendations for the treatment of discordant human circadian rhythms and associated conditions with the aforementioned countermeasures. Three research trials were conducted, two in the built environment of CFS Alert, and one on land during the SARTech Arctic Survival Course. During all three trials, subjects wore motion logging devices (Actigraphs), which measure ambient light as well as motion. Sleep data obtained from the Actigraphs was used to model the cognitive effectiveness of each subject. Furthermore, saliva was collected at regular intervals to measure melatonin and assess DLMO. In general, sleep duration was found to be significantly different between the January and June data collections at CFS Alert, with subjects in June sleeping an average of 46 minutes less than their January counterparts each day. Sleep duration was also found to fall significantly from Sunday to Monday for subjects in both January and June, resulting in reduced cognitive effectiveness in many of the subjects through the week. Circadian stress and poor cognitive effectiveness was most pronounced in the meteorology technicians that we studied, which we attribute to their variable work schedules.