"The great thing about owning your own company is that it doesn't matter which 80 hours a week you work."

That comment, from Doreen Garrett, the founder of Otis Technology, struck a chord with me when I interviewed her for my book Women on Top. Being an entrepreneur does give you great flexibility--but most business owners work almost all the time. And that's worrying, because vast amounts of research show that prolonged overwork is unproductive at least--and destructive at worst.

Personal Productivity

After working around 40 hours a week, we can all keep going--but, because we're tired, we start to make mistakes. We need extra time to correct those mistakes. So now we've just created a trap: more hours, more mistakes, so more hours.

The way out of the trap is to stop, get rested--and then resume work. How many of us feel we have the self control to do that?

The long hours have a cumulative effect on your productivity. Working for 11 or more hours a day more than doubles the risk of depression and a loss in cognitive function. You may have difficulty finding the right words, thought processes and problem-solving abilities become impaired, you're slower to react, and the ability to be creative gets harder to retrieve. Executives who work three or more hours longer than a normal, seven-hour day run a 60% higher risk of heart-related problems.

Feeling anxious? So did I when I read this research. And it has changed the way I work. Now, when I travel (which I do a lot) I schedule in recovery time. I book time for myself at the gym or a pool. I go to bed earlier and never allow screens, phones, or computers into the bedroom. Most of all, I stop when I'm tired. It's more efficient to go to bed than to persevere.

Engineers responsible for important machinery talk about asset integrity: maintaining and repairing vital equipment before it breaks. In the work that most of us do, that equipment is our brains. Seen from this perspective, it's every business owner's most precious asset. Doesn't that mean you should look after it?