Researchers tracked the health of 7,095 British civil servants aged between 39 and 62 over an 11-year period. During the course of the study, 192 poor souls suffered a heart attack; having crunched the numbers, they concluded that if you work an 11-hour day, as opposed to a standard 7-8 hours a day, you're 67% more likely to suffer heart problems. Sounds pretty clear-cut, right?
Well, there are a few caveats. First, this study was conducted entirely within the public sector - which isn't necessarily directly comparable with the private sector in terms of external or internal pressures (or with the third sector or entrepreneurs/ small business owners, for that matter). For much of the period, these civil servants had a level of job security that others could only dream of; although things have obviously changed hugely in the last year or so.
There's also no mention (as far as we know) of whether the scientists corrected for levels of seniority: a CEO working an 11 hour day and a stockroom assistant working for the same amount of time are likely to experience very different stress levels. It must also be incredibly difficult to correct reliably for other external risk factors like diet, lifestyle, and so on.
That's not to say we should dismiss the findings out of hand, of course. We can very easily believe that working long hours, particularly in a very stressful job, is probably not great for you. So it makes sense to look at this along with other risk factors. A bit of hard work might not kill us, but too much of it might. Perhaps we should all clock off at 5.30 today...