Workaholics are 'unproductive,' said millionaire investor Kevin O'Leary. He believes that great employees should have balance in their lives.

• People who work "25 hours a day" are unproductive, said investor Kevin O'Leary.
• The Shark Tank star told CNBC Make that he doesn't want to hire workaholics.

Kevin O'Leary, the influential investor and star of CNBC's Shark Tank, has some advice for anyone wanting to have a productive career; don't overwork.

"If all you do is worry about your job and where you are in the pecking order and just spend 25 hours a day working, you will be a less productive person,'' O'Leary told CNBC Make It, in a recent interview.

That is not a "fruitful" way for someone who wants to be productive at work to spend their time, he said. Instead, anyone who wants to be focused should spend half of their time not working, O'Leary said.

"When I see a workaholic, I don't want to hire that person," he said.

In 2021, a joint report by the World Health Organization and International Labor Organization estimated that overwork contributed to 745,000 deaths globally in 2016. People who consistently worked longer than 55 hours a week were at greater risk of heart disease and stroke.

American workers clock up an average of 1,767 hours a year, according to the latest (2020) data from the OECD. For many, the pandemic is likely to have increased the working day, with staff shortages, layoffs, and the boundary-blurring impact of working from home making it harder to shut off.

Despite its negative impact on our health and relationships, workaholism remains ingrained in modern business culture.

O'Leary himself previously told Insider that true work-life balance is a myth for any entrepreneur growing a business, until they have the financial freedom to do what they wish. As a result, he and his wife initiated a rule to spend time with family at the weekend, regardless of where they're working.

"I always look for someone to have some other talent other than numbers, because it actually gives that balance," O'Leary said. "Great" entrepreneurs, employees, and managers have balance in their life, O'Leary told CNBC Make It.

"Really good outcomes in terms of how I've ended up in business and as an investor came from ideas I had while I was playing my guitar, or polishing my watch. It lets your mind drift, and it lets you become more productive," he told CNBC Make It.

Anyone wanting to get ahead at work should keep a journal of their achievements, O'Leary said

O'Leary is the host of CNBC's Money Court and chairman of O'Shares ETFs. He doesn't shy away from providing anecdotes and insight on how to climb the career ladder.

One "green flag" he looks for in job candidates is when they include hobbies on their résumé because it shows that a candidate has balance, he previously said.

Anyone wanting a promotion should keep a journal of their achievements that they can use to negotiate with their manager, he told CNBC Make It last year.