The logic of only having 'essential work' done at nights because it is bad for your health just escapes me.
DH has gone out for the 5th night in a row to do real essential work, on the side of a motorway, overnight. It is essential, it is dangerous, as is a lot of overnight work - Railtrack and the London Underground use the, possibly disappearing, down time to do all cleaning and repairs, DH will be doing that next week.
So, by the previous posts logic, a shop worker shouldn't work over night, cos it would be bad for their health but, I am guessing, no one would want their travel infrastructure to close during the day for maintenance, each and ever day, forever.