All right, so let's say the work day increases by four hours once no one has to sleep. Most people now spend around twelve hours working, and twelve hours off.
Surprisingly, I don't expect too many changes to occur here, so long as people re flexible (so, basically, it'll be really bad for a few years, but I'll get to that). The first change that must occur is that salaries must be dropped, across the board. If people are getting paid the same amount but working 50% more, layoffs are going to happen, everywhere. Some industries will be worse than others (for instance, software developers will be hit hard since their software will be the same, while waiters might get saved with more nighttime shifts), but overall you've essentially got two people doing as much work as three people before this change, so there's no reason to keep all three people on the payroll. Not only that, but the people who get to keep their jobs are going to get more money, while the people who lose them get less, eventually leading to an even larger divide between the rich and the poor.
So everyone is now getting paid less hourly, but the same overall. There is a slight problem that comes from the fact that people will want more money when they don't need to sleep: keeping the lights on all night, every night will eventually outstrip the cost of a bed, as will the extra cost of food to offset the calories you burn walking around when you used to be sleeping. The effect of this will probably be that everyone seems poorer, which, again, shouldn't be a problem. After all, most people tend to spend the money they're given; you buy more stuff when you get a raise, and you buy less stuff when you get a demotion. There may be an adjustment period, when people go bankrupt because they're spending more than they can afford, but after a while people will get used to it.
So, like I said, as long as people change with the times, there shouldn't be much economic impact. The initial change is going to be the worst, and if things don't adapt there's likely to be a depression, but hopefully the transition is gradual or controlled enough to avoid that.