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.
It's as if the circadian system is in one time zone eastward in women compared to that of men.
Diane B. Boivin, MD
Although this study didn't find a clear relationship between participants' menstrual cycles and their sleep, previous research from the same lab showed that particularly during the mid-luteal phase of a woman's cycle (a.k.a after ovulation but before your period) REM sleep may be reduced. But that effect may have been too weak to pick up in this small study. So what's going on here? Well, your circadian rhythm is controlled, in large part, by a bundle of neurons in your brain called the suprachiasmatic nucleus. This area regulates the amount of sleep- and wakefulness-modulating hormones (such as melatonin) your brain releases. Those hormones then affect a bunch of other processes in your body. But Dr. Boivin explains that this area also contains receptors for female sex-related hormones, such as estrogen. Those hormones also regulate the menstrual cycle and the effects that come with it (including changes in core body temperature). So the same chemicals responsible for your cycle also interact with your internal clock. "Basically, the part of the brain responsible for the circadian system and sleep mechanisms has a sex," she says. It's not totally surprising that a gender difference like this exists: "It's clear there are sex differences in circadian rhythms," says Dr. Boivin, "and they are so fundamental to [so many processes in the body]." Indeed, those rhythms matter for way more than just your (very important) sleep. For instance, they can also affect the way your body processes some types of medication. But researchers are only beginning to understand how these mechanisms really work and when the gender differences matter most. That's partially because sleep studies — and health studies in general — often don't include enough women (if any) to make gender-specific conclusions. The current research is a start, but more is needed to confirm the effect outside of the lab. The participants in this study were all healthy and didn't report any sleep disorders, so the timing differences didn't have a huge effect here. "But we think this is the mechanism by which women [who are already susceptible to sleep problems] can develop sleep disturbances," explains Dr. Boivin. And, according to this study, that's especially true for staying asleep at the end of the night. Dr. Boivin advises women who may have sleep problems to avoid light exposure during the night. That means getting curtains that block light effectively — and no checking your phone.