I found taking my B12 at night helped me sleep! However, the Bcomplex, of all the B's tends to jive people up. (I think it is the folate that does it, myself).

B12 is used to make melatonin from serotonin.

Fatigue can be from other reasons. I am doing a trial of d-ribose and the details are on this thread:

http://neurotalk.psychcentral.com/thread123731.html

Since I have started the ribose, I have much more physical stamina and better muscle functions. So far I am very pleased with it.

I guess I was slipping into not being as active as I once was. This may be metabolic for me, with insulin resistance, or part of my arthritis issues, etc. Then there is the PN to deal with and the hypothyroid. So I thought the ribose looked promising for helping the old sagging mitochondria out. And it is turning out that way for me.

B12 is stored in the liver. Skipping for a little while after being on it for a long time should not result in a severe reaction, one would think. But there can be an issue called "vitamin dependency". This is where the body gets used to very high doses of a vitamin and then changes to require it. It happens with B6 (people are born with this and it is called infantile spasms/seizures) and extremely high Vit C intake ala Linus Pauling's old recommendations often led to scurvy when dropped down to normal intake levels suddenly. While the literature does not include B12 as a inducer of vitamin dependency, it still is possible I think for it to occur.