The circadian rhythm system is most responsive to light–producing phase shifts–when it is done therapeutically at the edges of a person’s “subjective night.” What is subjective night? It’s not the same thing as night outdoors. People vary by as much as six hours (early types, late types; larks, owls) in their subjective night. How can you find out about your subjective night and the specific time that morning light therapy is expected to work best for you (by phase advancing your internal clock)? Complete our Automated Morningness-Eveningness Questionnaire at www.cet.org, and you’ll find the answer.