So the combination of melatonin, smell and sound creates the conditions for being sleepy.

My own theory has to do with preservation of energy. If it rains most animals have a harder time to foridge for food or you can even get sick if you prance around in the rain. 

If you can't eat y...

Rain sounds are also comforting. The rhythmic pitter-pattern of rain hitting a roof, umbrella, or the ground below is called pink noise. Pink noise is a category of background noise that, like white noise, has all of the frequencies that humans can hear, ranging from 20 Hz to 20,...

The absence sunlight triggers our bodies to stop producing the hormone melatonin, which makes us sleepy at night. When it’s raining and skies are overcast, we miss out on our body’s internal alarm clock. 

Naomi Rogers, a chronobiologist from Central Queensland University, t...

Also there is the smell of ozone - a molecule made of three oxygen atoms, generated when lightning interacts with the air, breaking up and recombining oxygen molecules with two atoms into chains of three instead.

It also creates a powerful association with rain.