A company that wants to operate 24/7 would need to hire for the night-shift. This would be based on demand, of course. Night-shift hours are probably lower since most customers go to sleep at night.

I don't see why hiring evening and night staff would be that different apart from training. It's probably a bit more difficult to find people that want to work the night shift... some companies get around this by rotating staff between the day and night shifts regularly to give everyone a chance to lead a normal life.