Why do we have seasonal mood changes?
Most of us feel better when the sun is shining - more cheerful and energetic. On grey, gloomy days, especially in winter, we tend to feel less enthusiastic, more inclined to stay indoors or even in bed, to do less work, to socialise less and to eat more. The reason for this is the change in the quality and quantity of light. As winter approaches, there are fewer daylight hours and so, by December, we often get up in the dark and come home from work or school in darkness. What's more, the shorter winter days don't have the same light intensity that we get in summer.
The cycle of light and dark determines our sleeping and waking patterns. Until the widespread use of electric light, people used to wake and get up with the dawn light and sleep when it became dark. In winter, people would sleep longer and be less active. Nowadays, we tend to override these natural rhythms and manipulate the hours of light and darkness to suit our sophisticated lifestyle. Many night-shift workers and jet-lagged air travellers suffer from disrupting their body clocks in this way.