GLASGOW, UNITED KINGDOM - OCTOBER 10: A man makes his way home from work on a bus as darkness falls on October 10, 2005 in Glasgow, Scotland. Seasonal affective disorder (SAD), or winter depression, is a mood disorder related to the change in the seasons and the resulting reduction of exposure to daylight. The end of British Summer time, when clocks go back one hour at the end of October, will see most people making their daily commute in darkness both ways. With winter nights stretching to 19 hours in the UK, and Scotland's often inclement weather, it is estimated that the "Winter Blues" can affect up to 20% of the population. (Photo by Christopher Furlong/Getty Images)