Twice a day. If I don't I feel so sick I can't function. But I haven't tried taking a whole tab at night to see how it works in the morning.
My doctor prescribed me diclegis which is the B6 and an antihistamine. Maybe it's just me, but it doesn't make me the slightest bit drowsy. I'm supposed to take it at bedtime because it can make you drowsy. Works to keep away the n/v though. Might check with your doctor to see if that's a possible solution for you? Good luck!
I took it only at night so it didn't matter if it made me crash, and it was still helpful in controlling daytime nausea. Maybe you could try that? Or are you taking it twice a day already?