The condition has been labelled seasonal affective disorder, or Sad, since the 1980s.
The distinction has been drawn for decades: low mood is common as daylight fades, while a minority face a sharper and more persistent illness through the colder months.
Sad is not the ordinary dip that comes with shorter days. It is a winter depression that, for those who have it, can be debilitating.
As the nights draw in, many people experience low mood. For a small proportion, a more intense depression develops through the winter months.
People looking for support can find other international helplines at befrienders.org.