Why does rain make you tired?

I knew it was going to rain today because that's what the weather man said. But, as I hit the snooze button a few more times to catch more Z's, my body knew that it was raining outside even before I opened my eyes. Why is that? I always find that I am more lethargic when it rains, that I get up later and it is much harder to open my eyes and get out of bed.

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:

From http://www.answerbag.com/q_view/428921:

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.

From http://answers.yahoo.com/question/index?qid=20080509052301AAf2p2t:

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.