Disclaimer: I am not a medical doctor. These are my personal opinions. You should not use the information on this web page for diagnosing or treating a medical or health condition on your own. All decisions regarding patient care should be made with your healthcare provider.
Has vitamin B-12 been proven to help CRDs? The short answer is no, but it's a complicated story.
The interest in B-12 for CRDs came about because of one patient. He was one of the earliest reported cases of N24 and the first one that appeared to be successfully treated . He was the first N24 patient treated at the US National Institute of Health. Basically what happened was, this patient was somewhat hypothyroid. Since low thyroid function can lead to B-12 deficiency he decided, in his own, to take a B-12 supplement and found that shortly after his N24 resolved and it came back if he stopped the B-12 The NIH doctors were not sure how widely applicable that finding was, or if it was a fluke or a placebo effect.
However doctors in Japan were very interested in the B-12 approach to N24 and tried it with several N24 patients with variable success. In one survey 2 out of 10 N24 patients were helped. There was some interest in Japan and the US about how it might work and some work was done on that. For example B-12 might increase the effect of light. However, eventually, double-blind placebo-controlled studies were done and in those B-12 was not significantly better than placebo [2,3]. There was a "trend" toward improvement with B-12 in one study, but a trend that does not reach statistical significance is, well...not significant. A total of 50 patients with DSPS and 6 with N24 were studied. The studies used a form of B-12 called methylcobalamin, which is believed to be more effective than the more common cyanocobalamin (although I think the original NIH patient used cyanocobalamin). But even this allegedly more effective form did not exceed placebo. The Japanese doctors decided B-12 was "therapeutically insufficient" as a single treatment.
Since there are a few cases of N24 that seemed to benefit from B-12 it can't be completely ruled out that some people with CRDs might benefit from taking B-12. So it might be worth trying if nothing else works. But given the failure of B-12 in placebo controlled studies I would not consider it is proven to help CRDs.
In general B-12 is viewed as an energizing substance. Some of this reputation may be undeserved. People with B-12 deficiency are certainly lacking in energy (due to anemia and nerve damage) and so giving them a shot of B-12 will certainly energize them. As a result it became common for doctors to give people B-12 as kind of a tonic to boost their energy. Since B-12 deficiency is not rare that often works. Whether it really does help do that in people who are not B-12 deficient to start with is not clear. It could be a placebo effect. But, deserved or not, it does have a reputation as an energizing substance.
In one study on normal subjects B-12 (as methylcobalamin) did lower melatonin levels and reduce sleep time, consistent with a stimulating effect. Another study showed intravenous B-12 to raise body temperature both day and night in normal volunteers, and also increase slow wave sleep, which usually results from higher body temperature [4,5].
In rats under free-running conditions B-12 increases sleep time and makes the period longer (i.e. makes N24 worse). And in a breed of dogs with narcolepsy B-12 causes cataplexy.
The usual oral dose of B-12 is 1,000mcg (1mg). This is actually far
more than the body needs which is more like 2-6 mcg. The reason for
the high dose is that a few people do not absorb B-12 well and so the
dosage is designed to take that into account. Very few people
actually need that high dose, but in most cases taking extra does no