To be fair you should be sleeping for 8 hours every 24H period, not exactly 8 hours straight. More recent science is showing that sleeping 8 hours straight isn't as beneficial as sets of rest periods, 2 sets of 4 hours. We'd probably evolve differently if our timescale was 48 hours, if the ratio of sun and moon were the same, but we'd probably still have the same ratio of rest. 1/3rd of the day.