Periods often begin during the night or in the early hours of the morning. A study showed that a significantly greater number of menstrual cycles (70.4%) commenced during the night or within the first four hours after waking. [^1] Many women report their periods starting between 11 pm and 4 am, although some experience their period beginning later in the morning after waking up. [^2]
This doesn't mean that periods never start during the day, but nighttime and early morning seem to be the most common times. This could be due to various factors, though definitive scientific explanations aren't fully established. However, it's important to remember that every woman's experience is unique, and the time of day menstruation begins varies.
[^1]: Analysis of the time of day of onset of menstruation. PubMed. https://pubmed.ncbi.nlm.nih.gov/3690465/
[^2]: What time of the day does your period starts? Reddit. https://www.reddit.com/r/Periods/comments/1e7x2h8/what_time_of_the_day_does_your_period_starts/
