A study in the Annals of Behavioral Medicine found that people who engaged in leisure activities were 34% less stressed and 18% less sad while enjoying their hobbies. Odds are good you don't need to be told that leisure activities are fun and make us feel good. The problem is, you may not have the time or money to pursue them.

But what if you could earn a substantial living from your hobby? That changes the game, and turns it into a money-making activity that you actually enjoy doing. But which hobbies result in the big paydays you're looking for? You might be surprised to learn that doing something like selling T-shirts is a viable means of generating income.

Here are 8 ways you can turn a hobby into a six-figure income that will make you feel great at the same time.

1. Sell Your Merchandise on Amazon

Sites like Threadless and Zazzle made it easy to start designing T-shirts and selling them to audiences online. But not every platform meets the demand for sellers looking for eager buyers. Amazon is one of the latest retailers to join the merchandise bandwagon, and it's turned into a huge payday for some entrepreneurs.

Neil Lassen and Todor Karlikov earned $150,000 in profit by selling T-shirts on Merch By Amazon. The pair had little to no knowledge of T-shirt design, but devoured YouTube tutorials and took their chance on the Amazon print-on-demand and order fulfillment service. It wasn't long before they were earning enough to pay some bills, and ultimately turned it into a six-figure income. Start your own T-shirt business by looking at what's trending in current events or on Twitter to start generating buzz.

2. Flex Your Social Influence

It's not exactly easy to just launch an online presence and turn it into a six-figure income, but it can be done - and there's no educational requirement to do it. Today's influencers aren't just selling ad space and using Google AdSense. Social influencers with large and loyal online followings charge a premium for everything from a Twitter campaign to sponsored posts on their websites.

YouTube vlogger Zoe Sugg, otherwise known as Zoella to her fans, used her online earnings to buy a £1 m five-bedroom mansion in Brighton at the age of 24. She charges over £20,000 a month to advertisers vying for product placement alongside her fashion tips.

Start growing your own followers by focusing on an area you know best, whether that's growth hacking tips, fixing cars or a fun hobby that you can talk about for hours on end.

3. Invest in Real Estate

Millennials are making waves by investing in real estate, but not actually living in it, or using it themselves. Travis Killian owns multiple single-family rental properties around the country, despite not living in any of them. He told the New York Times he was looking at it as long-term income he could depend on.

Meanwhile, investing in commercial real estate was once reserved for multi-million dollar investors. Crowdsourcing real estate deals changed the industry forever, and lowered the bar for entry. Sites like Realty Mogul allow qualified investors to get started with just $5,000.

Look for real estate deals in areas you have a long-term interest in, whether that's office space and storage or the hotel industry. As you work your way forward, you'll have a hefty portfolio of commercial deals to show you're a serious investor.

4. Sell Photography

Ben Hartley launched an award-winning international wedding photography business and earned six figures in less than two years. He regularly interviews other photographers who went from amateur to six-figure pro in rapid time to find out how they did it.

Selling photography can encompass different industries and fields, from corporate to weddings, but it takes premium service and pricing to earn a six-figure income. Start with an area you know best to get your feet wet, and work your way into a lucrative field to grow the income you're looking for.

5. Share Your Voice

Voice-over artists are making it big in some unexpected places. Linnea Sage turned her voice into a six-figure income by selling micro-services on Fiverr. The platform charges $5 for small, introductory services and sellers can upsell on everything from delivery time to additional work. Sage made six figures for two years running by lending her voice for commercials, voicemail messages and YouTube explainer videos.

Anyone can sign up on Fiverr and start selling, but it takes some time to get the ball rolling and get in those first crucial reviews. New sellers should take some time to read through the Fiverr forums and tutorials to find out how to best showcase their gig, as well as research which sell the best.

6. Become an Amateur Bed and Breakfast Owner

Renting out part of your home, an extra property or even a room can bring in more than just some side income. Superhosts on Airbnb turned their homes into their own cottage industries and raked in six-figure incomes.

Pol McCann turned his Sydney, Australia property into an Airbnb money-maker and soon had it booked almost every day of the month. After six months of renting his place, he used the proceeds to put down a deposit on another apartment across the street and now earns $100,000 after all of his costs. Airbnb renters say phenomenal photos and great customer service are key to becoming superhosts.

7. Start Freelance Writing

Freelance writing has always been an easy side hustle for anyone looking to make some spare change. Or you can leverage it into a six-figure income by focusing on lucrative markets. Carol Tice of Make a Living Writing earned six figures as a freelance writer with no college education. She grew her popular blog into its own money-making endeavor when she launched courses and memberships catering to aspiring freelancers looking to make real money writing.

The trick is finding fields that are lucrative and in high demand. For example, learning the art of content marketing can explode your career with the need for high-end, well-researched content with viral potential. Business writing, healthcare and technical writing are also money-makers with the potential for six-figure earnings.

8. Run Fitness Bootcamps

Working out while getting paid to do it is a dream job for fitness fanatics. Turn the dream into a profitable reality by charging for your services. Brandon LaVack went from earning $35,000 to $150,000 a year as a personal trainer. He focused on marketing himself better, getting his clients results and continuing his education as a business owner and trainer to reach his income goals.

Working one-on-one with clients is the slow way to success and requires longer hours. Instead, run group boot camps at local parks or market your services to corporate clients looking for team-building exercises or wellness programs. Hire other trainers to work your bootcamps for you to diversify and earn more; just remember to ask for non-compete contracts so they can't take your hard-earned clients with them.

What about you? What are some of your favorite hobbies that you've scaled into big paydays? Let me know by leaving a comment below: