Adults need at least 2.4 micrograms of vitamin B-12 per day for forming DNA and healthy red blood cells and for proper brain function. It is also sometimes used for treating sleep disorders, although it may not be beneficial for this purpose, according to MedlinePlus.
Research results are mixed on the effectiveness of vitamin B-12 for improving sleep. A study published in "Neuropsychopharmacology" in 1996 found that one type of vitamin B-12, methylcobalamin, was beneficial for sleep problems because it affected melatonin levels, helping to regulate the sleep-wake cycle. The other type of vitamin B-12 tested, cyanocobalamin, didn't have the same effect. However, another study, published in "Psychiatry and Clinical Neurosciences" in 1997, didn't find a beneficial effect for methycobalamin. The second study was a higher quality study, since it was a double-blind, placebo-controlled study rather than an open study like the first, and it used a larger group of people for the study. Both studies used the same amount, 3 milligrams, of vitamin B-12.