The reason dehydration affects us more than men? “It’s because of lowered blood volume,” explains Dr Stacy Sims, an exercise physiologist. “When your oestrogen and progesterone levels are high [during the luteal/premenstrual phase of your cycle], you lose around eight percent of plasma volume – that’s the watery part of the blood.” And the higher progesterone levels also cause a resting rise in core body temperature, which can make you start to feel tired sooner, as well as decrease your tolerance for heat. That’s where sodium or salt levels come into play. Sodium helps transport water into the blood, but those elevated progesterone levels make this harder. Progesterone fights for the same receptors as aldosterone (the hormone responsible for regulating sodium), which increases the amount of sodium your body kicks out. And if you’re on the Pill or other hormonal birth control, the oestrogen and progesterone in your system can be as much as six to eight times higher. Feeling thirsty yet?