Could it be the soothing sounds of rain drops? Or the darkness that the clouds overhead cast?
What causes this sleepiness spell?
So, I googled this phenomenon and here are some answers that made sense:
The pressure changes when it is raining, for example thats how a barometer works. The changed pressure makes you sleepy. This is true, because even if its just about to rain, you are still sleepy.
sunshine lifts the mood and its effect on the body promotes the production of endorphins , and that makes you happy
Sunlight is needed to produce Vitamin D3 and it also produces Melanin. Also it's like how you get cold when your tired at night.