Sleep problems are distressingly common — approximately 40% of Americans report not getting enough ZZZs. And these issues are even more common among women than men. However, new research has found that, for many, the root of these issues may be that women operate on a slightly different circadian rhythm.First off, to truly appreciate the meaning of this, you need to know that your circadian rhythm isn't just when your internal clock tells your body when to wake up and fall asleep. In fact, many processes, such as your body temperature, operate on (about) 24-hour cycles, also known as a "biological day." These patterns of variation in your sleep, alertness, body temperature, and so on are all controlled by an array of chemicals and environmental signals (e.g. sunlight).For this study , published online today in the Proceedings of The National Academy of Sciences, researchers had 15 men and 11 women (including eight studied at two different points during their menstrual cycles) sleep in tightly controlled conditions for three days. During that time, participants alternated between several hour-long periods of being awake and being told to nap. That way, the researchers could see how participants responded to sleep cues throughout an entire biological day. The researchers tracked participants' core body temperatures, sleep quality, and melatonin levels in addition to how long it took them to fall asleep and how alert they were when awake.Results showed that, although men and women went through similar circadian-related changes throughout the day, women did so slightly faster than men. They tended to fall asleep and wake up earlier than men: "The tendency to fall asleep during a day is about two hours earlier in women than in men," explains Diane B. Boivin , MD, PhD, of the Douglas Mental Health University Institute and McGill University, lead author on the study. "It's as if the circadian system is one time zone eastward in women compared to that of men."This is also consistent with previous research showing that women are more likely than men to have biological days that are shorter than 24 hours.Plus, women reported feeling more tired than men when forced to stay awake at night. Women also had more difficulty staying asleep and were more likely to wake up "too early" than men were, which corresponded to changes in core body temperature and hormone levels throughout the circadian rhythm. These fluctuations are totally normal, and most of us won't have any issues. But this pattern does suggest that it may be easier for women than men to get out of their regular sleep routines and develop problems.