Yes, however they aren't clearly delineated, which is the point OP is trying to make. You can have categories, but you can't assign individuals to them because they have lots of overlap.
But this "aren't clearly delineated" gets pedantic sometimes. The same issue is true for assigning hours into morning/day/evening/night, yet no-one talks about issues there. Stratifying something continuous into categories suddenly becomes bad when talking about ethnicity.
That's because the hours don't get personally affected by their classification. I suspect it would suddenly be a big deal if the minimum wage for day vs. night work was different.
And anyways, is 4am morning or night? You'll get different answers in a city compared to on a farm, for example.