It seems like starting the next day at sunrise is far more natural. At what point did we start considering midnight to be the dividing point between days? Are there parts of the world where they don't share this practice?
hi! not discouraging other responses, but FYI you'll find some info in the FAQ (link on sidebar):
Various cultures have measured a day from sunrise to sunrise, from sunset to sunset, from noon to noon, and from midnight to midnight. This has been commented on at least as far back as by Pliny the Elder source. In modern day Swahili speaking East Africa, time is measured from sunrise and sunset, so saa 1 is one hour after sunrise or sunset (as distinct from English usage, where 1 o'clock is one hour after either noon or midnight). For religious purposes, Jewish days are considered to run from sunset to sunset, and so Jewish holidays begin in the evening of what others might consider the prior evening.
Speculatively, one might note that in a society without clocks, sunrise and sunset are more of an obvious event than midnight or noon. Judging that the sun is at the horizon is easy, noting that it's at its zenith is a bit tougher, and recognizing that the night is exactly halfway done is even harder. This makes sunrise or sunset seem a more attractive choice. The advantage of midnight and noon is that they are relatively fixed throughout the seasons, whereas in non-equatorial latitudes, sunrise and sunset will drift considerably over the seasons. Using midnight or noon is thus a more attractive choice the further a society is from the equator.