Whether it's selling jewelry on Etsy or driving for Uber on the weekends, nearly 40 percent of Americans – including more than half of Millennials – have a side hustle, according to a new report.

A majority (59 percent) of those who have a side gig on top of their regular job do it to have extra cash on hand, while 38 percent said they have an extra job just to make ends meet, according to a survey by Bankrate.com.

'People do side gigs for all kinds of reasons and economics is only one of those reasons,' said Diane Mulcahy, author of 'The Gig Economy' and adjunct lecturer at Babson College. 'For the most part, particularly among Millennials, it's become a kind of expected normal thing that they do. I work a lot in the entrepreneurial space and it's very much the norm that you do some side gigs.'

The majority of people with a side gig make $200 or less in a typical month, while 17 percent make up to 500, and 15 percent make more than $1,000

Mulcahy said that in her research she has talked to many people who had a side gig for a specific goal – such as a vacation or to give their kids a big Christmas, while others care less about the money and more about making themselves more employable in the long-term.

'People also take side gigs even if it doesn't pay well or pay at all because they want to develop skills they don't have, or expand their network, or pursuing something that they're interested in,' she said.

Differences emerge between genders and political parties: Republicans are 8 percentage points more likely than Democrats to have a side gig for extra cash. Men are also 7 percentage points more likely than women to have a second income.

Men also tend to make more at their side hustles, earning an average of $989 per month at a side hustle - nearly three times as much women, who average $361 per month. That could be because men report working more often than women – 14 percent said they're pursuing a side gig weekly, compared to just 9 percent of women.

'Studies have shown men do much more negotiation of their incomes than women do and that trend may also be true in the gig economy,' Mulcahy said.

Overall, the most common side jobs are home repairs, landscaping, online sales, crafts or childcare, according to the survey by Bankrate.com.

Three-quarters of people who have an extra job do it at least once a month, earning, on average, an extra $686 a month – or $8,232 a year.

Perhaps unsurprisingly, Millennials are more likely than older generations to have a side hustle, or be interested in the opportunity to pursue one.

Part of the attraction for young people may be the freedom that comes with side jobs and freelancing, said Archie Ravishankar, CEO and founder of Cogni, a mobile banking platform focused on serving gig economy workers.

'The future of work, I've always believed, is that you don't have to be stuck in a single place,' he said. 'So this creates a new economy where people are using creative talent to generate the cash flow and also pursue a career that's much more creatively independent.'

The biggest single side hustle reported was home repair, with 12 percent of survey respondents. Crafts and online sales follow, with each attracting 7 percent of people who have a side job

Seattle resident Caroline Brown has been able to carve a career almost exclusively out of side gigs.

'I am constantly hustling,' she said. 'The freedom and flexibility is great … I really get to steer my schedule, which is wonderful. Also, I'm a multitasker, so having my hands in a lot of different baskets is where I really thrive.'

Among her many jobs, Brown teaches community-based arts and film directing and acting at two different colleges in Seattle, as well as community-based arts at a non-profit program serving adults who have experienced trauma and mental health issues.

She also has a contract position where she trains arts organizations to work with underserved populations – including people who have survived trauma, are living with mental health issues and who have experienced homelessness. Brown has also had stints working in interior design and as a bartender.

Working in the gig economy has allowed her to pursue her passion for the arts and helping people. But the life of a hustler is not always easy.

'I would say 30 percent of my job is chasing my pay checks, reminding people that I sent invoices,' she said. 'It's having to spend hours on the phone with HR to find out where a check has been mailed ... As much as the flexibility can be pretty exciting and offer a lot of freedom, it's also just so much upkeep.'