When in doubt don't take it. Under certain conditions, too much metformin can cause a serious condition called lactic acidosis, a life threatening condition where oxygen levels drop to dangerous levels. Set an alarm clock, ask a family member to remind you, place it in small dish next to you where you sit at night.
How much do you take? might be a relevant factor.
I once took my night time met twice, (my normal nighttime does is 1000) anyways I called the hospital and spoke with the ER they called an MD and I was told to not worry about it.
the dose which u take should taken after or just before meal