40 percent of American workers will be independent contractors by 2020

When Leversey Henry gets ready for work, he turns to his phone.

He uses an on-demand staffing app to look for available shifts in his area, but not in just one industry.

"I've done warehouse; I've done some product marketing, as well as event coordination," said Henry.

Henry answered questions on the app regarding qualifications and background ahead of time. Then, whenever a gig pops up, it's his for the taking.

"I actually use it as my primary source of income," said Henry.

There are several on-demand apps--offering gigs in industries that used to be so-called traditional jobs, in areas like retail and restaurants.

One survey predicts there will be more than seven and a half million on-demand workers by the year 2020.

A.J. Brustein is the co-founder of one on-demand staffing app. He says the employer-employee dynamic is shifting.

"I think for too long companies have had all of the control and haven't listened to what the workers really need. Today, we see flexibility in becoming the number one thing people are looking for and the gig economy is, you know, allowing that to happen," said Brustein.

Henry says he likes the flexibility and control over when he works for whom and the variety of opportunities.

"And then, you know, the quick turnaround in terms of how you get paid," said Henry.

Sometimes you get paid the same day. There can be some challenges, though, with a shift-based lifestyle.

"If you're looking for any sorts of things to watch out for as a worker, one of them, I would say, is probably just stability or reliability," said Brustein.

He says you have to be more active in your job search if you want daily work.

While some online forums slam shift work, saying the trend takes away from regular jobs. Brustein does not buy it.

"We're finding that traditional employers will use gig work to sort of try people out. So we often find that rather than spending a bunch of time on going through resumes or looking through job boards, they'll just bring people in and see how they perform and if they do a good job, off them a permanent position," said Brustein.