I recently started taking a B-12 supplement, as someone had recommended it for some pain I've been having. I just realized that since then, I've been sleeping really soundly, which normally does not happen, especially when going thru emotional turbulence like I've had this week.
I did some research and it turns out that B-12 deficiency can manifest as depression, sleep disturbances, fatigue, personality changes, there are links to Bipolar, alzheimer's, dementia, and lot of other things.
A vegetarian diet, drinking, smoking, many prescription drugs can all cause lowered b-12 absorption.
Here's one of the sites I found. There's a lot more information out there as well. Thought I'd share!
Vitamin B12 deficiency - Wikipedia, the free encyclopedia