Driving for Uber or selling handmade jewelry on Etsy can be a great way to fill in the gaps in your budget.

Before you pick up a "side hustle," though, make sure it won't affect your main source of income.

To that point, 44 million Americans have some kind of side job or extra source of income in addition to their primary work, according to a survey by Bankrate.com released Wednesday. Younger millennials age 18 to 26 are the most likely age group to do so, the survey found.

It makes sense that millennials would dominate the side-hustle population, said Bankrate's Sarah Berger. They have grown up in the digital age and are more familiar with the apps and technology many gig economy start-ups use. Plus, they have a financial incentive.

"For a generation known for drowning in student debt, it's necessary for millennials to be financially free faster," Berger said.

Before you start that side hustle, it's worth checking to see if there are any restrictions from your primary employer. The terms of your contract or the company's conflict-of-interest policies may limit your options, or if you can have one at all, said Alison Green, blogger for Ask a Manager.