Rainy days often mean more time spent indoors. Dark clouds, windswept streets and precipitation invite a more introspective attitude as well. A variety of studies show how weather affects your personality and mood; there are biological reasons why rain makes you feel sleepy and less energetic.
When it rains, the sky gets black with heavy clouds. Morning and afternoon can look more like early evening. Humans' biological response to darkness is to close their eyes. The body's internal clock, taking cues from the environment, initiates the sleep cycle.
The rain creates a soothing sound drowning out the shrillness of an urban environment, with traffic and construction sounds muffled. In a more pastoral setting, birds stop chirping and other forms of wildlife take refuge. The pitter-patter of rain has a rhythm and pattern, like a lullaby. This triggers a physiological response of drowsiness.
When it is sunny outside people absorb vitamin D through their skin. At healthy levels, vitamin D contributes to proper bodily functions. People who live in rainy climates all the time or otherwise have limited exposure to sunlight often take supplements. In these cases rain has been found to lead to chronic fatigue.
Sunshine is linked to activity, whether it is playing a game of toss or rushing to a work assignment. People feel guilty if their peers are outside and they are not. Movement produces an ongoing adrenalin rush. Food is eaten on the go. When it is raining aside, people tend to be more sedentary, consume more and generally slow the pace. This all contributes to a feeling of being tired.