Dr Emma Ross is a female health specialist and physiologist at Jennis
, a workout app with specialist programmes for pregnancy, postnatal and general training. She says that while there is general knowledge of periods and the existence of hormones, "the important thing that we don't necessarily tune into is how our hormones fluctuate." Over the course of a typical cycle, you can go from both oestrogen and progesterone being very low during your period to oestrogen peaking in your follicular phase before both hormones are elevated in the luteal phase, then drop off at the end. That fluctuation affects our ability to reproduce and either get pregnant or have a period, but it is far from the only thing it affects. "Our body has receptors for those hormones, all across our physiology: in our muscles, in our heart, in our bone, in our brain, in our gut. So when those hormones are high or low, they're going to affect loads of different physiological systems and they can affect metabolism, or they might affect mood, which might affect energy levels."