I have to agree with Stubborn on this...has she been checked for Sleep Apnea or anything like that. Wanting to sleep this much and falling asleep at a drop of the hat IS NOT NORMAL.
And it still could be depression. Clinical depression needs to be treated and could go on for YEARS if not treated. I would really suggest that you encourage her to go get checked. It would be sad if you could have a much happier R and the solution were this simple and it wasn't checked out.
Look at it this way...what do you have to loose? And if nothing else you're ruling something out if it turns out not to be the case.
