Perimenopause is the transitional phase leading up to menopause, often beginning in a woman’s 40s, though it can start earlier. During this time, hormone levels—especially estrogen and progesterone—fluctuate unpredictably, leading to symptoms such as irregular periods, mood swings, sleep disruptions, brain fog, anxiety, hot flashes, and more. These changes can last for several years before periods fully stop and menopause is reached.