What constitutes a "normal" night's sleep?
That all depends, says Danny Forger, a biological mathematician at the
"People set guidelines all the time -- you need eight hours, you need seven hours -- but we've found that 'normal' sleep varies tremendously depending on your age, sex and what country you are in," he said.
Forger is the senior author of a new study that analyzed the sleep habits of more than 5,000 users of a mobile phone app known as ENTRAIN.
The free app was designed by Olivia Walch, a graduate student in Forger's group. It was launched in 2014 to help users efficiently overcome jet lag with the help of a complex algorithm based on Forger's previous research. Essentially, the app tells people when to expose themselves to light and when to avoid it to minimize the effects of jet lag.
Entrainment is the scientific term for fully adjusting to a new time zone -- hence the app's name.
However, the group also invited users to send the sleep data they plugged into the app back to the servers at the university. Roughly 8% of the app's international users agreed to share their information anonymously, providing the team a treasure trove of real world sleep statistics.
"Lab conditions in sleep studies are very regulated, and participants don't have to deal with the stresses of the real world like spouses, children and all the stuff that keeps us up at night worrying," Forger said. "Also, in the lab, it would cost about $20,000 per individual to get this type of information and we don't have that kind of budget."
In a paper published Friday in Science Advances, Forger and his co-authors used the data they collected from ENTRAIN to provide an overview of the social and solar cues that influence when we go to sleep and when we wake up.
Most people in the data set schedule between seven and eight hours of sleep a night, with a mean of 7.88 hours.
Of all the factors considered in their analysis, gender plays the biggest role in how long a person sleeps. On average, women schedule 8.07 hours of sleep, while men schedule 7.77 hours. Women both go to bed a bit earlier than men and wake up a bit later. The authors say this effect is most pronounced among people 30 to 60.
Age also seems to be an important factor for when people sleep. On average, older people schedule sleep earlier than younger people. Also, there is less variation in the times that older people sleep then younger people. The authors think this could be because older people are more sensitive to solar cues, and can only sleep during certain times of the days. Younger people, such as college students, don't seem to have those restrictions.
Nationality also plays a role in sleep duration. Residents of Singapore and Japan had the shortest sleep duration of the 20 countries represented in the study, getting an average of 7 hours and 24 minutes of shut-eye a night. People in the Netherlands were the most well-rested, averaging 8 hours and 12 minutes of sleep a night. In the United States, the average sleep duration is 7.87 hours.
The authors also note that despite black-out curtains, artificial lights and alarm clocks, solar cues -- the time of sunrise and sunset -- still have a significant effect on sleep patterns.
"We recognize that these solar effects are dulled compared to predictive models, but the fact that they are still there in the data was pretty shocking to me," said Amy Cochran, an assistant professor at Univeristy of Michigan who also worked on the study.
Both sunset and sunrise had a bigger effect on when people woke up, compared to when they went to sleep. Later sunrises (after 6:30 a.m.) were associated with later wake times and bedtimes. Later sunsets were also associated with later wake times and bedtimes, but the effect of sunset on what time a person went to bed at night was weaker than what models predicted.
The authors conjecture that the time we wake up is regulated by our internal, biological clocks but the time we go to sleep is more influenced by societal pressure to stay up.
"We didn't expect to see that, since most people use their alarm clocks to wake up," Forger said. "It could be that people are hitting snooze. They try to set their alarm clock for 5 a.m., but it doesn't work for them."
But that's just a guess. Forger said the ENTRAIN data set is full of unanswered questions as well as trends they would like to see validated in the lab.
In the meantime, the researchers are also hoping to soon collect even more detailed data from ENTRAIN users. They are launching a new version of the app shortly that will take advantage of information collected by activity trackers that monitor both exercise and sleep.
"It is a much richer data set, and we will be able to rely less on self-reported data," Forger said.
He added that a large-scale sleep study such as this one could never have been done without the help of citizen scientists willing to share their sleep data.
"It is really a new way of doing science," he said.