A reader asks:

 Do most people respond to light therapy even at the “right time” for phase advances or delays? Does it need to be done at late night or early morning when it is dark?


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 and you’ll find the answer.