Our species spent most of its evolutionary history in equatorial regions, where dawn and dusk came gradually at much the same time all year around. Countless centuries later, our brain is still geared to expect a similar daily pattern of light and darkness, even though we have largely migrated to latitudes where the days are longer in summer and shorter in winter. This displaced position on earth still overwhelms the influence of man-made interferences: streetlights, room lights, and late-night TV and computer light. Human migratory patterns have produced predictable but often unrecognized effects on our mood, energy, and sleep.

One such effect presses on us now as we move toward winter. Winter blues and its more severe relative, seasonal affective disorder (SAD), are marching in. The symptoms of SAD — gloom and despair, excessive sleep, food cravings, pervasive anxiety, among others — add up to a major depression. Yet there is one big difference from unpredictable depressions: its timing, with the clear link to light deprivation in winter. Getting too little light, or getting enough but not at the right point in the day, puts you at risk for up to half a year of relative agony.

Fortunately, the cause implies a cure. Getting enough light, and at the right time, offers a way of dealing with the problems created by light deprivation. By restructuring the amount and pattern of our daily light exposure, we can achieve remarkable therapeutic results, even in cases of severe depression or sleep disorders. Best of all, the intervention is environmental, not pharmaceutical. The most widely known method is bright light therapy. This involves sitting in front of a therapeutic light box of a particular brightness, for a particular length of time, at a specific time of day (usually in the early morning). Each user needs to experiment at the start to find his or her optimum settings.

The therapeutic technology is evolving, bringing the light signal more toward the equatorial ideal. Of course, daylight and nighttime do not turn on and off with the flick of a switch. Rather, the eyes and brain received the signals gradually, at levels well below full daylight.