Contributed by Sebastian Bailey, PhD, Co-Founder and President, Mind Gym, @DrSebBailey

As far as first 100 days go, unexpectedly increasing revenue and the share price of a company described as “ailing” while raising a newborn is not a bad result for Yahoo! CEO Marissa Mayer.

The fact that Yahoo! appointed Mayer as CEO in July should not have come as a shock given her success at Google, except for the fact that she was pregnant—enough to make many directors think twice. Luckily Yahoo! didn’t fall for “baby-brain” stereotypes and made the right decision.

Mayer’s return to work two weeks after giving birth certainly highlighted her commitment to her new role. It also reignited the “how much is enough” maternity leave debate. There are oodles of opinions but little empirical research. Studies linking maternal employment with childhood obesity are accused of adding to working parents’ guilt. New research suggests that it’s not the amount of time parents spend at work that matters, but the amount of psychological strain they’re under.

In a study of 359 working mothers and their children, researchers Johnson and Allen examined the link between mothers’ work, their physical activity and their children’s health. They measured the number of hours mothers spent at work and their psychological strain, in the form of role ambiguity (being unsure of what is expected of them) and the amount of control they had over their work.

Unsurprisingly, mothers who exercised more were more likely to have physically active children who were healthier. Although “I haven’t got enough time to exercise” is a common cry, the researchers didn’t find a significant link between the number of hours worked and mothers’ physical activity. But there was a link between psychological strain and exercise. According to the scientists, strains like being unsure of what’s expected and not having control drains employees’ resources and leaves them with less energy for exercise. This, in turn, means their children are less likely to be physically active and therefore less healthy.

The psychological strain theory may explain why some people, Marissa Mayer being a great example, seem to have infinite resources to take on the corporate world and parenthood simultaneously. There’s no ambiguity for Mayer—her job is to turn the company around. And as CEO, there’s little doubt about who’s in control. Mayer herself acknowledges that “you can’t have everything you want, but you can have the things that really matter to you. And thinking that way empowers you to work really hard for a really long period of time.”

So what hope is there for the mothers who aren’t in a seven-figure-salary chief executive role? Employees and employers both have responsibilities to reduce psychological strain. Clear goals followed up with regular performance conversations ensure there’s no confusion about what’s expected. Employees can increase their sense of control by identifying the aspects of the role they can influence. By being aware of and avoiding the things that drain you, you can reserve energy for an evening jog or making that Halloween costume—whatever it is that really matters.