Then they may go elsewhere, to someone not on a 9-5 monday-friday culture. Many now space people out (ie some on a w-th weekend) so the company can provide 7-day service at normal rates. I used to work in film/tv. 7-day service is a norm.
24-hour is harder, but when you have clients across different time zones, it is also a norm. Again, with big films scattering themselves across the globe, it is expected that someone in authority picks up 24/7 because your 3am is sometimes the client's 10am.
You can charge more for this, but the client doesn't want to hear complaints. You will be judged on service rendered for a given price. The firm down the street, the one who can provide people on a sunday without overtime, will win the next contract.