Previous research has analyzed variations in sleep patterns including number of hours slept, quality of sleep, and sleep-wake times, and found an association with cognitive impairments, health and performance; however, few studies have considered or accurately quantified the effects of regular sleep patterns. In a new study at Brigham and Women's Hospital, researchers objectively measured sleep and circadian rhythms, and the association to academic performance in college students, finding that irregular patterns of sleep and wakefulness correlated with lower grade point average, delayed sleep/wake timing, and delayed release of the sleep-promoting hormone melatonin. The results are published in Scientific Reports on June 12, 2017.

Researchers studied 61 full-time undergraduates from Harvard College for 30 days using sleep diaries. They quantified sleep regularity using the sleep regularity index (SRI), a newly devised metric. Researchers examined the relationship between the SRI, sleep duration, distribution of sleep across the day, and academic performance during one semester.

"Our results indicate that going to sleep and waking up at approximately the same time is as important as the number of hours one sleeps," stated Andrew J. K. Phillips, PhD, biophysicist at the Division of Sleep and Circadian Disorders, Brigham and Women's Hospital and lead author on the paper. "Sleep regularity is a potentially important and modifiable factor independent from sleep duration," Phillips said.

Students with more regular sleep patterns had better school grades on average. Researchers found no significant difference in average sleep duration between most students with irregular sleep patterns and most regular sleepers.

"We found that the body clock was shifted nearly three hours later in students with irregular schedules as compared to those who slept at more consistent times each night, stated Charles A. Czeisler, PhD, MD, Director of the Sleep Health Institute at Brigham and Women's Hospital, and senior author on the paper. "For the students whose sleep and wake times were inconsistent, classes and exams that were scheduled for 9 a.m. were therefore occurring at 6am according to their body clock, at a time when performance is impaired. Ironically, they didn't save any time because in the end they slept just as much as those on a more regular schedule."

By measuring the timing of melatonin release at sleep onset, the researchers were able to assess the timing of circadian rhythms. On average, melatonin was released 2.6 hours later in students with the most irregular sleep patterns, compared to students with more regular sleep patterns.

"Using a mathematical model of the circadian clock, we were able to demonstrate that the difference in circadian timing between students with the most irregular sleep patterns and students with regular sleep patterns was consistent with their different patterns of daily light exposure," stated Phillips. "In particular, regular sleepers got significantly higher light levels during the daytime, and significantly lower light levels at night than irregular sleepers who slept more during daytime hours and less during nighttime hours."

Researchers note that the circadian clock takes time to adjust to schedule changes, and is highly sensitive to patterns of light exposure. Irregular sleepers, who frequently changed the pattern of when they slept and consequently their pattern of light-dark exposure, experienced misalignment between the circadian system and the sleep-wake cycle.

Researchers conclude that light based interventions, including increased exposure to daytime light and less exposure to electronic light-emitting devices before bedtime, may be effective in improving sleep regularity.