Why Is My Period Heavier At Night? Your Period Follows Your Circadian Rhythm, Too

Ethan Miller/Getty Images News/Getty Images

If you're a person who menstruates, you may notice that your period doesn't behave the same way in the morning as it does at night. Your period may be heavier at night, or your flow might be more intense in the morning. The reason why your period changes over the course of the day is an interesting one, and it's related to how human activity shifts over the course of a day and night — what's called the body's circadian rhythms. The menstrual cycle, which has a roughly 28-day run time, isn't the only time scale your period is affected by: it also varies over the course of 24 hours.

The body has its own internal "clock," regulated by a combination of things including hormones, which dictates how it reacts throughout the day, prompting sleepiness and wakeful behavior. And that also has an impact on how your menstruation works, too, from how your body's hormones react to how you behave. Everybody's period is slightly different, so there's not one way in which people are affected by the passage of 24 hours while they're having their periods. But the idea of menstrual bleeding shifting at different points, it turns out, doesn't actually have to do with the level of bleeding itself, but rather with how your body shifts during night and day.

24 Hours In The Life Of A Period

The short answer to whether or not you bleed more, or less, at night is that most people do not. Sleep variations in period flow are often, Dr. Michelle Petropoulos points out in a post for UKotex, pretty illusory, and caused by the shift in our behavior at night. Lying down means that gravity no longer works to move period blood from the uterus through the body in the same way, which means that blood may "collect" outside the body (which makes it appear as if the flow has been heavy) or inside it (which creates the impression of lesser flow). There isn't any serious scientific evidence to suggest that variation of flow occurs from sleep to waking, but we do know that flow levels vary in menstruating women from day to day.

24 hours in the life of a period can vary pretty considerably in other ways, too, and not all of them are due to external factors like body position. In the pre-menstrual phase of your cycle, it's known that you're more at risk for sleep disturbances, and that carries on into your period, meaning you may wake up a bit sleep deprived. A 2003 study also found that during our periods, women's levels of the hormone estradiol, which are highest at this point in the cycle, peak later in the morning than at other times. There's some evidence that estradiol levels are associated with mood swings and depression, which may be why you experience some mid-morning blues as you bleed. And the energy you expend during the day is also influenced by whether or not you're menstruating (it seems that women both eat more and burn more energy during their luteal phase, the part of the cycle just before menstruation).

These sorts of ideas about how menstruation relates to the body's internal "clock" are just beginning to be explored. So why would circadian rhythms affect your period at all?

The Circadian Rhythms & Your Period

Looking at bleeding and other menstruation symptoms over the course of a day of your period can be pretty revealing. In the case of bleeding, they show that the idea of heavier or lighter flow while sleeping is a myth, but they can also demonstrate other interesting things about female health. Anna Druet, a research scientist at the period app Clue, which tracks period data from millions of users to get new insights about menstruation, explains on Clue's blog that circadian rhythms and your menstrual cycle are pretty connected:

"The circadian rhythm and menstrual cycles are influencing each other all the time. If you picture a spinning top moving in a big circle, each little spin of the top is the circadian rhythm, and each big circle is the menstrual cycle. If the spinning top gets out of balance, the size and shape of the big circle may change. The top also spins a little bit differently depending on where it is in the big circle."

This is particularly notable in cases where your circadian rhythm is thrown off or adjusted, as when you suddenly shift your sleep cycle or get jet lag. But it's also got other effects on the way periods work.

Research in 2007 found that circadian rhythms are tied to periods in many different ways, particularly in how well women sleep and in their body temperature. The scientists also noted that menstrual function appears to be "thrown off" when women's sleep-wake rhythms are disturbed; shift workers are apparently more likely than other women to report irregular periods, for example. And the hormones that regulate the body's sleep seem to shift over the menstrual cycle and from day to night; women's brain activity in sleep shifts over the course of their cycles, with the most activity just before your period starts. Women also, according to science from 1988, have different temperature shifts throughout the day depending on their time in their cycle (temperature has a lot to do with how we feel sleepy), and heart rate might shift as well.

So while your levels of blood flow might not be impacted in any way by your sleep-wake cycle and how you act over 24 hours (though it's pretty clear how that misunderstanding can arise), many other bits of menstruation are affected. And that's helping us get a bit more insight into how the female body works. Pretty cool, huh.