Absence of Circadian Phase Resetting in Response to Bright Light Behind the Knees

Light is the dominant environmental time cue for circadian clocks. In 1998, bright, narrow-spec-trum blue light exposure to the back of the knees was reported to reset the human circadian pace-maker (HCP) (1). Science recognized the widely cited report as among the top discoveries that year to “transform our ideas about the natural world ” and reported that several groups had repeated the finding (2). Patented treatments for circadian sleep disorders followed (3, 4). Yet the report was challenged be-cause humoral phototransduction via the circulatory system, which was cit-ed as a mechanism that might mediate such a circadian resetting response