Consider the Great Recession: Americans slept the most during 2009, 2010 and 2011, when unemployment was high. That probably wasn’t because stress was low or attitudes about sleep were healthier — it was because fewer people were working. Or this fact: Retired people sleep more than working age people, even though the biological need for sleep actually declines in old age.

“The major determinant of short sleep is actually work,” said Mathias Basner, a professor at the University of Pennsylvania’s medical school, who has studied short sleepers. “People who work a lot of hours, they are much more prone to be short sleepers.”

Other activities that appear to be disproportionately displacing the time of short sleepers: commuting, television watching at night and personal grooming in the morning, according to Mr. Basner’s work. If you’re trying to sleep more without cutting back on work hours, Mr. Basner recommends avoiding TV before bed, spending less time getting ready for work and living closer to your workplace.