The FINANCIAL -- For all the power CEOs wield, they can't add hours on the clock or days to the calendar.

Yet everyone wants a piece of them: Leadership teams, senior managers, rank-and-file workers, customers, board members, investors, outside groups, the media. And they want their attention face-to-face, on email, by text and by phone, even on weekends and vacations.

Add to those demands CEOs' personal needs — for rest, relaxation, time with family, exercise and doctor appointments.

That's why those running companies have to be very discerning in how they use their hours.

On average, CEOs work 62.5 hours a week, according to Michael Porter, director of the Institute for Strategy and Competitiveness at Harvard Business School, and Nitin Nohria, dean of the school.

They've been conducting a long-term study on how CEOs spend their time. They ask CEOs to keep track of how they spend every hour of every day for three months. So far, they have 60,000 hours of data from 27 CEOs of large, mostly public companies.

The study also found that CEOs spend about 4 hours a day working on most weekends; and about 2.5 hours a day during most vacations.

But how they use their hours determines how successful they'll be.

Here are some of their recommendations:

With meetings, shorter is sweeter

CEOs reported that about a third of their meetings lasted an hour on average, while 38% went longer.

Make time to be alone

CEOs are supposed to set a vision for their companies, lay out broad strategies to realize it, and clearly communicate both to stakeholders.

That's hard to do without adequate time to reflect. On average CEOs spent 28% of their work time alone, but the majority of it was in blocks of an hour or less.

Delegate more

Getting too involved in the day-to-day can derail CEOs from putting their time where it's needed most.

For example, with reviews, CEOs can become too mired in company operations, Porter and Nohria noted.

Use email less

CEOs reported spending 61% of their work time in face-to-face interactions, and 24% on electronic communications.

While email can be an efficient way to communicate, it can also be a time suck.

Having an executive assistant filter and delegate them before you even see them can help.

Don't schedule every minute

CEOs in the study spent about a quarter of their time on average on spontaneous interactions.

That's critical, the authors assert, so executives can be available for opportune conversations or meetings and to deal with unfolding, unexpected events, which consumed about 36% of CEOs' time.



Keep in touch with employees

CEOs spent about 14% of their time with low-level managers and 6% with rank-and-file workers.

That's time well spent.



It also helps CEOs better understand their employees' experiences, which pays dividends down the road, they noted.



Make time for your life

CEOs in the study seemed to get the memo that they needed to rest and stay healthy. They slept nearly seven hours a night on average. About 9% (or 45 minutes) of their non-work time was devoted to exercise.

And of the 25% of their time (or six hours a day) spent awake but not working, half was time spent with family.