How much sleep did you get last night? Are you a night owl or an early riser—or both?

For college students, the answer appears to be the tougher your school, the later you go to bed. (Yale students averaged a 1:07 a.m. bedtime on weekdays and a 1:35 a.m. bedtime on weekends.) And most students appear to be getting about seven hours of sleep a night.

How do we know this? Data collection through wearable devices—in this case, the wristband activity tracker Jawbone UP. It measures to the minute physical activity such as running or walking, but it also tracks sleep times.

The activity data is uploaded to the cloud, where you can then check out how you’re doing. But the company can also use the combined personal data, stripped of identifying characteristics, to examine trends and posit interesting conclusions and general trends.

For instance, in the above study, Jawbone took average bedtimes and compared them to the often-cited U.S. News and World Report college rankings. The schools considered the toughest also showed the latest average bedtimes.

Yet a couple of these tough schools looked like anomalies, with average bedtimes before midnight. Slackers? Quite the opposite: These were the students at the U.S. military academies, and most of them woke up before 7 a.m., with a sleep time of an average 6.4 hours.

The uses and limitations of sleep data

Sleep—how long, how deep, and at what times—has become a big issue in health and wellness, with CEOs and innovators praising its importance and Arianna Huffington authoring a book on the topic: She calls a good night’s sleep a “leadership tool, a performance enhancement tool.”

Sleep expert Dr. Meir Kryger, Yale University School of Medicine, agrees. When Huffington’s #SleepRevolution tour came to Yale, he shared his 13 Commandments of Better Sleep (turn down your phone brightness!) with an audience of several hundred students, some of whom reported sleeping as little as three hours the night before. Dr. Kryger’s books and blog hold even more insights about the power of sleep.

The sleep issue is so important to health and learning that the Yale College Council, the student government for undergraduates, identified getting enough sleep as a core project for the 2016-2017 academic year.

Likewise, wearable devices and other data collection methods can be important tools for improving student life, health, and wellbeing. But before we all rush to buy a sleep-monitoring bracelet and alter our personal habits or institutional policies, it’s a good idea to keep the limitations of data in mind.

We all still need to read the signs intelligently: Knowing military academies are tough schools and require earlier wake-up calls is essential to making sense of the sleep study.

Jawbone itself points out that while interesting, its report is far from the full picture. Students who wear activity trackers might have better financial resources or be healthier overall, with better sleep habits (getting seven hours, even though they do stay up late). Students who don’t get enough sleep might be doing so because of jobs or family responsibilities. And not every student would choose to opt into a system that tracks their every move—many are rightly sensitive about personal data privacy.

The takeaway here: Lack of sleep is a perennial issue off campus and on.  Using data to help students—and ourselves—understand and get healthy sleep habits is a plus. But we can’t close our eyes to the challenges presented in collecting, analyzing, and implementing that data. Big data can play a big role in improving student life and student services. But we’ll have to be careful that it doesn’t get so big that it eclipses the importance of individual student’s lives.