Is it safe to run on your period?
For the large majority of runners, yes, and it can genuinely help. Easy running tends to ease cramps, lifts mood, and there is no physiological reason to stop training just because you are bleeding. Many women are surprised to find the first days of a period feel better than the week before it, as estrogen starts to climb again.
Why some period days feel great and others feel awful
The bleed itself is only part of it. The rough days are often the late luteal and PMS stretch just before, when both hormones drop. Low iron from heavy bleeding can leave runs feeling flat and heavy. And poor sleep the night before will color any run. So "running on your period" is rarely one feeling; it is a few different days that deserve different sessions.
How to run well on your period
Train to how you feel
Do not assume the week is a write-off, and do not force a key workout on a day you clearly cannot execute it. If you feel strong, use it. If you feel flat, an easy run still counts. Phaes builds this in: it reads your daily check-in instead of forcing a calendar.
Fuel iron-aware
Bleeding plus training is where iron slips, and low iron is one of the most common reasons running feels harder than it should. Phaes keeps fueling guidance iron-aware through your bleed.
Do not write off the whole week
Plenty of personal bests have been set on a period. Cycle-based training is not about doing less when you bleed; it is about putting hard work where you can absorb it. See how cycle-based training works.
When to back off, and when to ask a doctor
If bleeding is very heavy, pain is severe, or you are persistently flat and exhausted, that is worth a conversation with a clinician, especially to check iron. Phaes is informational and not medical advice, but it will not push you to grind through a day your body is clearly asking you to ease.