There is new information out in Dec 08 that discusses this
relationship.
You can read the article here:
http://neurotalk.psychcentral.com/forum53.html

Melatonin is made in the body from serotonin using methylB12 as a cofactor.

Now that we know that there are melatonin receptors on the pancreas, there may be new treatments for preventing diabetes!

And keeping serotonin up and making sure you have enough methylB12 will help as well.
I think we will see much more research into this.

And BTW... SSRIs do NOT increase serotonin in the brain.
In fact they lower it. This is because the cells cut back on manufacture when reuptake slows. This is why people have
side effects when they try to discontinue. Also the metabolic by products of serotonin go down in the spinal fluid-- one clue that production is lower.
http://www.nature.com/npp/journal/v2.../1300025a.html

Serotonin is made from l-tryptophan using B6 as a cofactor.
Then the melatonin is made from serotonin using methylB12.

So--- poor sleep= lower melatonin which may lead to type II diabetes, which then leads to PN. This is an interesting chain of events, dependent on some nutrients, and normal sleep cycles.