Email support is not like telephone support where you can have office hours.
For big companies maybe, but for small one man companies there are certainly hours in which they are reachable, at least things like sales. Support should be available as often as possible, but people running a one man webhost do need sleep
It also depends on what you are offering. People paying the least amount of money usually expect the most for it. That is, they expect to get velvet ropes, the red carpet and a dedicated support tech that replies to their trouble instantly for the 1 dollar a month package they got last month.
Not to say I don't want to do all I can to keep my customers happy, but some do stretch the limits of what you can/want to do.