The most obvious explanation for the shift is that our economic requirements have evolved in favour of daytime pencil pushers. Maybe we do still need some night-time workers to taxi us to early flights or tend to our wounds in the wee hours, and perhaps there will always be pressure for eager beavers to pull all-nighters. (A study of Danish white-collar workers found that night-time work boosted their chances of promotion.) But perhaps demands elsewhere — and for employees competent enough to do their work in the allotted time — have been stronger.