There is a study that some people get winter 'blues' (or Seasonal Affective Disorder) from the lack of sunlight, grey skies, etc - particularly people who live in the northern hemisphere where they have long periods of night.
I wonder if even in more temperate climates, closer to the equator were sunshine is not a problem whether by living with your curtains/blinds closed all the time it can have the same effect.
The first thing I do when I wake up is open the blinds in all the rooms, open windows and doors, even if it is cold and raining (obviously not if the rain is coming in though). At work, it is the same - the blinds are always opened when I walk in every morning, in fact our whole end of the building has the same philosophy.
Surrounding ourselves with light seems to make sense if we want to feel happy. How do you let the light into your day?