Most people associate the hours 8am - 12pm with AM and 12am - 4pm with PM. This is because many people go to work in the morning (8-12am is when the working day normally starts) and get off in the afternoon/evening (4-8pm is when many workplaces end their working day). Furthermore, many people have lunch around 12-2pm and plan to get home around 4-6pm, so they often associate 12pm-4pm with PM.