High doses of vitamin B12 have been reported to relieve a few cases of DSPS and non-24-hour sleep-wake syndrome. Vitamin B12 neither helps you wake up nor fall asleep in the short term, and it is not yet clear why vitamin B12 would have an effect on circadian rhythms.
Reportedly, the dose needed for a significant effect in treating these circadian disorders is 2-3 mg per day, which is about 1000 times the ordinary recommended daily allowance. There have been no reported side effects from such large doses of vitamin B12. Vitamin B12 tablets are available from many drugstores and health food stores. The vitamin is also available in injectable form, but that is only for people who cannot absorb it at all from food.