Whether you look at polls of employees or Silicon Valley's obsession with perks like unlimited vacation days, it's clear that flexibility is the latest hot workplace benefit. Some commentators have even gone so far as to call flexibility the new definition of success.

There's plenty of reason to cheer this development. Studies show that flexibility reduces stress and can boost productivity, and on a human level having the ability to adjust your schedule to help an ailing relative or attend your kid's field trip is certainly nice. But while there's a compelling case to be made for workplace flexibility, that doesn't mean the idea is all upside, as two new studies illustrate.

Want your employees to work more? Give them flexibility.

The first downside of flexibility will come as no shock to anyone who has ever made the switch from working for someone else to being your own boss. Just because you set your own hours doesn't mean you'll work a humane schedule. In fact, people seem to drive themselves harder when given control over their time, as British sociologist Heejung Chung recently explained in Quartz:

We examined data that followed workers across a number of years in Germany to see what happened to the amount of overtime they did once they started having more control over their working hours.

We found that this tendency for people to work more when given greater control held true even when we took into account a whole range of factors that influence your likelihood to work longer, including level of authority and type of job. And this increase in working hours was greatest when workers had full autonomy over their working hours.

Earlier research in United States came to similar conclusions: If you want your people to work more, give them flexibility. That might be an tantalizing finding for bosses, but for workers it suggests that setting your own hours, with all its benefits, also raises the chances of burnout.

Setting your own schedule takes mental energy.

Workers might put in longer hours when they set their own schedule, but given the ability to adjust their routine based on their energy levels and life snafus, they're working more productively too, right? Actually, maybe not. A huge new Harvard study (also written up in Quartz) paints a more complicated picture.

The study examined more than two million records from a firm that pays doctors to examine patient scans like x-rays. It found that the more flexibly the doctor's worked (i.e. the more they deviated from the firm's suggestion that they review the scans in the order they are received), the less they got done.

Why is that of interest to anyone other than hospital administrators? Because it reveals another drawback of setting your own schedule: Thinking through when and how to work actually takes a significant amount of mental energy that ends up diverted from work tasks. In short, when you work flexibly, you spend time deciding what to work on when that time could be spent actually working.

"Searching through your queue and deciding which task to choose next may not seem like it, but it's actually taking you a long time," commented María R. Ibáñez, a doctoral candidate at Harvard Business School and the lead author of the study.

The bottom line on flexibility.

So taking all this evidence together, is flexibility an all-around win or loss for employee productivity and happiness? The answer probably depends on the kind of work an employee does (who cares that a creative gets a little less done if the quality of the work is much better, for instance), his or her personal life constraints and character, and the nature of the business.

Flexibility involves tradeoffs. Workers tend to love it, but they can burn themselves out if not given guidance. Productivity on some kinds of knowledge work probably goes up while routine tasks may take longer unless tightly controlled. The best solution depends on context, but to arrive at the right policy for your team, it's important to know that flexibility isn't all roses and kittens -- like any other policy, it has significant downsides.