Under the Weather

I`m feeling under the weather today. Now why do we use that expression I wonder?
Is that that the state of the weather has an effect on us? I know that a bright or sunny day usually makes me feel cheerful and more confident to do things, whereas if I have to tackle something I`m not sure about on a grey dull day it seems so much harder to do the same thing.
But does the weather really have any effect on our actual health?
So why say ‘under the weather’ for not feeling well? I`ve just looked up the definition of that expression and come across that it`s said to originate from the fact that an inexperienced sailor who feels seasick shelters from the wind by crouching down beside the bulwarks – “under” their protection – on the “weather” side of the ship.