Studies of human circadian rhythms are typically conducted in artificial environments that are low in ecological validity. In the current study, six subjects and the field director lived in temporal isolation in a completely natural environment with constant daylight (a high Arctic research camp) for six weeks. Detailed daily sleep logs were kept. In keeping with past findings, five of the six subjects developed a free-running sleep-wake cycle longer than 24 hours. Unlike past results, the isolated subjects did not exhibit any synchronicity in their rhythins. There was a high degree of intersubject variability in circadian patterns. The findings have important implications for the comparison of the results of laboratory and field investigations of sleep-wake cycles.