I slide it over my head until it rests just above my eyes and switch it on. For the next half-hour, two thumb-sized bulbs that fit into the underside of the visor pour light 20 times brighter than the lighting in my house into my eyes.

The recuperative powers of light have been recognized for thousands of years. One Greek physician recommended that "lethargics . . . be laid in the light and exposed to the rays of the sun." A ship's physician on an 1889 Antarctic expedition noted that expedition members were afflicted with "languor" during the winter darkness and that "bright artificial lights relieve this to some extent." And in the early 1900s, J.H. Kellogg treated more than 5,000 patients a year suffering from a variety of ailments with phototherapy at his Michigan sanitarium. But in the ensuing years, light therapy fell into disuse among the mainstream medical profession.

One of the largest studies of the use of light to treat clinical depression was published in 1992 in the journal Biological Psychiatry. Researchers led by Daniel F. Kripke, a psychiatrist at the University of California at San Diego, administered bright white light treatment to 25 depressed hospitalized patients at a Veterans Affairs Hospital and compared them with 26 patients who were exposed to dim red light, which has no effect on depression. Both treatments lasted one week and none of the patients in either group was taking psychiatric drugs.