Answer by Steven Fowkes:

B12 participates in methylation reactions, along with folate, B6, B3, B2 and methyl donors (TMG, choline, etc.). Do you have an unusual response to any of these?

B12 is a fermented vitamin. There are microbial forms of B12 that are anti-metabolites for humans. These anti-metabolites are supposed to be cleaned out by purification of the fermentation, but maybe they were not in this particular case. If so, switching brands could provide a different response.

B12 is known to produce vivid, vibrant and hyperactive dreams, sometimes even erotic and psychotic. Although this can be entertaining on an occasional basis, it can be exhausting when it is repeated or chronic.

It is also possible that you have an accumulated sleep deficit and the B12 is only enhancing a normal sleep process, or sabotaging a sleep-compensation process (e.g., adrenal activation). If so, the sleepiness effect should attenuate rapidly as you catch up on your sleep (assuming that you lower the dose so as to attenuate excessively vivid dreaming, too).

It is possible that you are merely stimulating melatonin. If so, bright blue-rich lighting might decrease the sleepiness. Blue light exposure in the middle of the day is one treatment for seasonal affective disorder and melatonin attenuation may be one of the associated mechanisms of this effect.

B12 is also an catabolic-aerobic-acidifying vitamin by the classification system of Revici (and acid-ash macrobiotic classification, too). So is B6; all the other Bs are anabolic-anaerobic-alkalinizing. Normally, a catabolic-aerobic-acidifying influence would wake you up and promote alertness. But your experience could be some kind of idiosyncratic response. If so, other unusual responses to similarly or oppositely classified nutrients might be experienced. Similar agents would include B6, magnesium, selenium, cysteine/glutathione, thiosulfate, vitamins D3 and A (not beta-carotene), and polyunsaturated oils (flax and fish). Opposite agents would include B1, B2, B3, B5, folate, salt, vitamins E and K, cholesterol, glycerol, fatty alcohols, and plant greens (especially sea veggies).

If you develop some insights into your question, please post them here.

Why does vitamin B12 make me sleepy?