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.