A reader asks:

Will longer exposure to a dimmer light box (say, 7500 lux) accomplish the same effect as 30 minutes at 10,000 lux?


The answer is probably yes, although how much longer is not clear. There has been no direct, systematic research on this question, although we know that light boxes providing 2500 to 10,000 lux can be fully effective given sufficient exposure duration. The required duration at any dose level is determined in part by the individual’s own sensitivity to the light. For example, even with 10,000 lux, one person may need only 20 minutes of exposure, while another needs 30 minutes or even 60 minutes. In early clinical trials of 2500 lux, the typical duration was 2 hours and above. One trial with 7000 lux settled on 1 hour exposures for all patients, although that may have been too long in some cases.