According to a mental health therapist, the rain makes white noise. It is define as a sound, especially of a continuous kind, that seems to have no pattern or rhythm. With white noise this will put you into a half sub- conscious state and will make you feel sleepy adding the dark sky above. And when we think about it, it is a sleepy environment with the wind and breeze cooling the environment.
That explains my friends in why we feel lazy and sleepy when it's raining. I hope you learn something as I am.