Melatonin is not an herb. It is a natural hormone your body makes when it is healthy and not being negatively impacted by the drugs doctors give you.

You won't have any if your B12 is low. You will mess it up by exposing yourself to light when you need to be asleep.

It is often used to help reset people with jet lag.

No, doctors won't tell you about it. If you want I'll spend some time and get you the documentation, but it is often here on the various NT forums over the years because this question comes up frequently. Losing sleep is very very common.

for example
http://www.ncbi.nlm.nih.gov/pubmed/9622603

http://en.wikipedia.org/wiki/Vitamin_B12

In fact, many doctors don't even know what methylcobalamin is.
They think cyano would work, and it will only do so if the patient can detoxify/remove the cyanide portion from it and methylate it in the body. Some people have a genetic failure and cannot do this function. Methylcobalamin is now available at low prices OTC. Use orally in 1 up to 5mg daily on an empty stomach is as good or better than cyano injections of B12.

This Wiki article explains the wavelengths of light that affect melatonin secretion in your brain:
http://en.wikipedia.org/wiki/Melatonin
It is very interesting!
There are sunglasses called BluBlockers which block blue light.
Melatonin is secreted from the pineal gland, which is not in the brainstem. It is just above it, as shown in this diagram:
http://en.wikipedia.org/wiki/File:Gray719.png