The study, published Monday in the journal Scientific Reports, says college students who did not go to bed or wake up at consistent times every day were more likely to have lower grades.
For the study, 61 students from Harvard College kept online diaries of their sleep schedules for 30 days. Researchers identified two groups: regular sleepers, or those who went to bed and woke up about the same time every day, and irregular sleepers, who had different sleep patterns every day.
There were several differences between regular and irregular sleepers, including significant differences in grade point averages. Using a unique scoring index from zero to 100 to calculate a student's sleep regularity, students with very irregular sleep patterns were given lower scores close to zero, while more regular sleepers were given higher scores close to 100. The researchers found that for every score increase of 10 on the regularity index, the student had an average increase of 0.10 in their GPA.
Andrew Phillips, lead author of the study and associate professor of medicine at Harvard Medical School, said the findings show that irregular sleepers have a delayed release of the sleep hormone melatonin.
"Our body contains a circadian clock, which helps to keep time for many biological functions," he said. "One of the key markers of the circadian clock is melatonin. Usually, at nighttime, our circadian clock sends a signal that tells us to release melatonin overnight."
Depending on what time melatonin is released, it helps to set both the sleep and wake cycles for our bodies. So with irregular sleepers, melatonin is released later in the night, pushing the circadian clock later as well. Phillips found that the irregular sleepers in his study had much later circadian rhythms -- by almost three hours, on average.
Dr. Charles Czeisler, another study author and chief of the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital in Boston, said that can make your body feel like it's in another time zone.
"That means that if the student had an 8 a.m. class, it would actually be happening at 5 a.m. biological time," he said. "It's as if they were traveling from the East Coast time zone to the Pacific time zone."
Irregular sleepers were also found to have less exposure to light during the day but more exposure at night, which also can affect the circadian clock, whether it's sunlight, fluorescent light in a classroom or artificial light from a phone or another electronic device.
"We know the human circadian clock is very sensitive to light. Your patterns of light exposure are also what set the timing of your body's circadian clock," Phillips said.
Kristen Knutson, an associate professor of sleep medicine at Northwestern University and expert in sleep patterns and their relation to health, said the study emphasized an aspect of sleep that people don't usually think about: regularity.
"Much attention has been paid to sleep quantity and quality, which are important as well," said Knutson, who was not involved in the new research. "But the importance of sleep doesn't end there. So, other factors like sleep timing and like sleep regularity are important to provide a more holistic perspective that includes all these dimensions of sleep health."
And the importance of sleep regularity is what Phillips and Czeisler want to emphasize, too.
"When circadian rhythms are disrupted, it degrades many different physiological systems in the body and makes individuals much more vulnerable to adverse health outcomes," Czeisler said. "That could be everything from catching a cold to gaining weight to diabetes, and here in this study, we're showing it can degrade academic performance as well."
But Knutson, as well as Phillips and Czeisler, notes that the study can't say irregular sleep definitely causes bad grades -- though it's very likely the factors are related.
"It's possible that students who have bad sleep habits also have other bad habits that are the reasons behind their poor grades," Knutson said. "That being said, if it is possible that irregular sleep could impair a student's ability to do well in school, we really need to understand this further."
So what can tired college students -- or those about to start college -- do to improve their sleep regularity?
First, decide on a reasonable sleeping and waking time, and then stick to it. Signing up for classes around the same time every day could help students to be consistent.
"It's so simple to address, that's why I call it a secret weapon," Czeisler said. "Regular sleep and adequate sleep is the best secret to student success."
It could also help to maintain a consistent sleeping schedule on the weekends -- and maybe not sleep in so much.
The study said there was no difference in the number of hours of sleep regular and irregular sleepers got.
Czeisler says that means an early bedtime is not a requirement for making your sleep pattern more regular: "The results of this study are not suggesting everybody has to be a goody-two-shoes. So if you go to bed at 2 and get up at 9, that's fine. You just have to consistently do the same thing."