There's no single reason why a period might start at night; it's primarily due to the complex interplay of hormonal fluctuations within the body. While it can seem like periods preferentially start at night, it's more accurate to say that they can start at any time, day or night, and the timing varies between cycles. A significant study found that a greater number of menstrual cycles (70.4%) actually commenced during the night or within the first four hours after waking. https://pubmed.ncbi.nlm.nih.gov/3690465/
Hormonal Influence
The timing of menstruation is significantly influenced by the body's hormonal changes. These fluctuating hormone levels trigger the shedding of the uterine lining, which manifests as a period. The exact moment this process begins isn't precisely predictable and may vary from cycle to cycle. [Why does my period start at night? The time that your period starts is influenced by hormonal changes in your body. It can start at any time, day or night, and can vary from cycle to cycle.]
Perceived Timing
It's important to note that the perception of a period starting at night might be skewed. Some individuals may simply notice the bleeding more readily when lying down at night due to gravity. [Is It Normal for My Period to Stop at Night? (for Teens) | Nemours ... https://kidshealth.org/en/teens/pm-period.html] In reality, the bleeding likely began sometime before it was noticed.
No Consistent Pattern
While some people report periods consistently starting at night, this isn't a universally experienced phenomenon, nor is there a scientifically established reason for this perceived pattern. Many factors influence the menstrual cycle, and the start time is just one aspect that lacks predictable consistency. [Why it's likely you'll start your period on a Thursday or Friday https://www.cosmopolitan.com/uk/body/health/a46604697/start-period-thursday-friday/]
In Summary
The start time of a period is primarily dictated by fluctuating hormone levels, and while it may seem to begin more often at night, this is likely due to a combination of hormonal processes and the increased likelihood of noticing bleeding while lying down. The actual commencement of menstruation isn't tied to a specific time of day.
