As winter approaches, its short dark days can cause some people to slip into a deep depression lasting anywhere from a couple of weeks to the entire winter. People suffering from this type of depression may have what's known as seasonal affective disorder, or SAD, a condition that develops from a combination of genetic vulnerability and inadequate light exposure, according to David Miklowitz, a University of Colorado at Boulder psychology professor. In Colorado, the peak period for SAD is November through March, he said.