In the summer, we wake up with the sun and go to bed when it gets dark, so our body's internal clock is in synch with the sun. But in the winter, most of us force ourselves to wake up while it's still dark. And that means that the sleep-wake circadian rhythm is out of step with the other rhythms, which we can't control with an alarm clock. Scientists suspect that light therapy helps fight winter depression because it kicks the other biological rhythms, namely melatonin, into step with our artificially manipulated sleep cycle. A study by Dr. Alfred Lewy, of Oregon Health Sciences University in Portland, supports that theory because it showed that morning light is more effective than evening light.
Bright lights suppress the brain's secretion of melatonin.
In other words, your hormones want you to stay sleeping, but the light convinces them that it's time to wake up. For roughly 80 percent of SAD patients, melatonin levels peak just when it's time to wake up. For them, morning light therapy is best. For others, melatonin drops too soon, and they find themselves waking up in the middle of the night. For those patients, evening light therapy might work better.