My neighbor's kids go to bed past 4 a.m., is it a cultural thing?

My apartment is located on the 2ond floor of 4 blocks of flats. On the 4th floor there's an Indian or Pakistani family (judging by the way they dress and their looks). The father is rarely at home, and the mother lives with three kids.

The thing is that the mother doesn't put them to bed until 4 am in the morning  (I hear them playing until then, and they're very loud). Another neighbor told me the kids don't get up until 1 pm and rarely go to school.

Do you know if this is a cultural thing or is that family clearly dysfunctional (or is the mother mentally ill?).