Is something starting to feel different? Many women first notice subtle shifts in their sleep, mood, energy, or menstrual cycle. These early changes are often the beginning of perimenopause — the hormonal transition that gradually leads toward menopause. This phase can begin in a woman’s late 30s or 40s and may last several years before periods stop entirely. During this time, fluctuations in estrogen and progesterone can affect sleep, mood, memory, libido, and overall sense of well-being.