"In the first half of your cycle, just after your period has ended, estrogen begins to increase, which means that your skin begins to rebalance itself," Ferguson says. "You might notice your skin gets its glow back and that your hair is strong and glossy. Vitamin C is a great ingredient to incorporate into your routine around this time, as your skin should be clearer and calmer. As well as providing antioxidant protection, vitamin C should boost and extend this radiance. This continues into the second week where you’re most fertile, energy levels are higher, and you tend to feel more positive."Experts say that vitamin C works best applied as part of a morning skin-care routine, as it fights against all the environmental aggressors (like pollution) we're exposed to during the day.