The saying is mainly about sleep. If you have to finish a paper by 8am you can stay up all night to finish it. This may seem like a brilliant move because, after all, what were you going to do with the hours between midnight and 7am anyway? You were just going to waste it in bed. So now your paper is done and all you missed was a night's sleep.
But all you've really done is borrowed time. You haven't stolen it. Because you stayed up all night on Thursday, you'll invariably crash on Friday. If not on Friday, then you'll sleep in an extra five hours on Saturday. If you barely catch up on sleep over the weekend, you'll likely get sick the next week. If you don't get sick and you keep pushing yourself on empty, your productivity will slide. Or you'll get into a car accident because you're so tired. Or you'll snap at your friends and cause a relational meltdown. All of which take time. You will have to make up for the seven hours of sleep you missed the week before. In fact, the longer you try to borrow against sleep, the more your body (or God) will force you to pay for those hours plus interest. That one all-nighter might cost you three full days of wasted time after all is said and done.
I recently read an article by a computer programmer about how making sleep a priority makes us more productive. There was nothing deep or spiritual about the post, but I imagine his experience, in general, has been the experience of a lot of folks, even if our details are quite different.
Three years ago I started a company in San Francisco with some friends. I didn't quit my day job, so this was an after-hours project. We set up an office and established a routine of working from 6 p.m. to midnight. After we started working for the new company full-time, I fell into a classic trap of San Francisco startup culture: I confused work hours with productivity. Somewhere in the back of my mind I knew that working smart was better than working hard, but I convinced myself that I was doing both.
I wasn't. As a programmer, I averaged 10-12 hours in front of a computer every day and rarely went to bed before midnight. Eventually, I felt dull and unmotivated. When I took two weeks off to travel in Colombia, I spent the first few nights sleeping for 10 hours each. I realized that if there is such a thing as sleep debt, I had accumulated some. I made up my mind to correct my sleep patterns.
I know this is easier said than done, especially for moms and dads with young children. But it would be worth your time (and mine!) to talk to your friends or spouse about how your life might be suffering from constant sleep deprivation. I doubt many of us will be able to solve the whole problem, but I doubt there isn't something all of us can do to make sleep a higher priority. Less caffeine at night, no internet past 9pm, no t.v. past 10pm, better planning during the day, scheduling your week so that known sleep-deprived nights will be followed with lighter days---whatever little things you can do to pay back your sleep debt will undoubtedly be good for you, your work, your soul, and the ones you love.