In the practice of yoga, it is customary to take a few days off your asana practice during the first few bleeding days of the menstrual cycle. But why do we do this? Do we have to?
One of the primary effects of Ashtanga Yoga practice over a long period of time is that it brings awareness of the Vayus, or the directional energy forces in the body. If this seems a little "woo," think about the way you feel heavier when you're ready to go to sleep, or the way you get a burst of upward energy when you are excited to go somewhere. (Or, like, just observe a child for a few hours, ha!) Directional energy is real and tangible.
Most of the asana practice stimulates prana vayu, or inward and upward energy (not to be confused with Prana, or Qi, that's another topic). This is balanced with actions that foster grounding (a couple of examples, the action in the legs in the standing poses, or the "nose" dristhti in backbends), but the overall effect is IN and UP. Cultivating this directional energy, according to the science of Ayurveda, has many benefits: concentration, connection, contentment, balanced energy and a strong immune system, to name a few.
BUT, when you are in the bleeding phase of your menstrual cycle, the directional energy that you want to cultivate is Apana Vayu, which is downward and out. Not entirely dissimilar to when you need to defecate. Can you imagine sucking your belly in and up when you need to poop? Yikes.
Over time, paying attention to the cycles that your body goes through can be really helpful in lots of ways. Anecdotally, I know many people, including myself, whose cycles have become more efficient (fewer days of bleeding), less painful, and more enjoyable (I know!) since they started taking a few days off Ashtanga practice when they bleed.
One last thing: Ashtanga Yoga isn't (or shouldn't be) dogmatic. It's experiential. Take a few months to observe your cycles. Meditate during that time instead of doing asana practice-- or do a different kind of movement, one that feels nourishing and calming. Restorative yoga with alllll the bolsters, sure! I had a wonderful teacher once who led us through the forward bends in the primary series but without the vinyasas and with props to make them restful-- sure! See what you discover. My recommendation would be two or three days, depending on how long you usually bleed. I take three days off from asana. If you are worried about losing progress on poses you're working on by taking a few days off, the only thing I can tell you is to try it and see what happens. Many of us Ashtangis tend to get pretty focused on results, and while that's not necessarily good or bad, sometimes it's unnecessary.
One last thing-- in the Western cultural paradigm there's a whole lot of good vs bad going on-- and we sometimes apply that to situations where the dichotomy doesn't make sense. Prana vayu is GOOD, so we should cultivate it all the time forever! Mulabandha ALL THE TIME!! No. This is a misunderstanding. Yoga and Ayurveda teach us to do the appropriate technique at the appropriate time, not label one technique good and one technique bad, and do only the good and cut out the bad. It's important to observe and honor our natural cycles.
Any follow-up questions?