Facts:
Warning, 300 mg per day for extended periods (several months in the published literature) has been clinically demostrated to cause neurological damage that persists after B6 useage has stopped and may be permanent.
As stated many times on this forum 2000 mg causes neurological damage in a single dose.
(the FDAs recommended maximum of 100 mg/day is there for a reason. While the FDA has been wrong many many... Many times its best to heed their advise until sufficient scientific evidence disproves their stance. One small clinical study found 250mg to be measurably superior to 100mg in inducing vivid dreams, but one should not do this daily)

Educated hypothesis:
In that the normal B6 pills contain pyridoxine (the biochemical precursor of active B6) instead of pyridoxal (B6) trying to take them while sleeping may be wasted effort because the liver must convert pyridoxine into pyridoxal before it can enter into the process of neurotransmitter biosynthesis, and this conversion is not instantaneous. This would explain why many people who take pyridoxine hydrochloride in the mornings or durring the day are frequently sucessful.