Combining math modeling, mobile apps and big data, a new study has parsed the roles society and biology each play in setting sleep schedules.
The study, led by University of Michigan mathematicians, used a free smartphone app that reduces jetlag to gather robust sleep data from thousands of people in 100 nations.
The researchers examined how age, gender, amount of light and home country affect the amount of shut-eye people around the globe get, when they go to bed, and when they wake up.
Among their findings is that cultural pressures can override natural circadian rhythms, with the effects showing up most markedly at bedtime. While morning responsibilities like work, kids and school play a role in wake-time, the researchers say they're not the only factor. Population-level trends agree with what they would expect from current knowledge of the circadian clock.
"Across the board, it appears that society governs bedtime and one's internal clock governs wake time, and a later bedtime is linked to a loss of sleep," said Daniel Forger, adding "At the same time, we found a strong wake-time effect from users' biological clocks, not just their alarm clocks. These findings help to quantify the tug-of-war between solar and social timekeeping."
The U-M researchers also found that middle-aged men get the least sleep, often getting less than the recommended 7 to 8 hours. Women schedule more sleep than men, about 30 minutes more on average. They go to bed a bit earlier and wake up later. This is most pronounced in ages between 30 and 60.
People who spend some time in the sunlight each day tend to go to bed earlier and get more sleep than those who spend most of their time in indoor light.
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Habits converge as we age.
Sleep schedules were more similar among the older-than-55 set than those younger than 30, which could be related to a narrowing window in which older individuals can fall and stay asleep.
Sleep is more important than a lot of people realize, the researchers say. Even if you get six hours a night, you're still building up a sleep debt, said co-author Olivia Walch.
"It doesn't take that many days of not getting enough sleep before you're functionally drunk," she said. "Researchers have figured out that being overly tired can have that effect. And what's terrifying at the same time is that people think they're performing tasks way better than they are. Your performance drops off but your perception of your performance doesn't."
Aside from the findings themselves, the researchers say the work demonstrates that mobile technology can be a reliable way to gather massive data sets at very low cost.
"This is a cool triumph of citizen science," Forger said.
The study appears in the journal Science Advances.