A new simulation by GrowFit Health shows what really happens to your body when you fast for 24 hours. Many people fast to trigger what experts call a “deep repair” — a process that helps the body clear damaged cells and boost metabolism.

After just 4 hours without food, insulin levels drop and your body starts burning stored sugar for energy. By 8 hours, your glycogen stores begin to deplete. At 12 hours, your body enters mini-ketosis, using fat for fuel. This can lead to weight loss, but also fatigue.

At 16 hours, a powerful process called autophagy begins. This helps clean out damaged cells, toxins, and even viruses. While autophagy starts earlier, fasting for 24 hours deepens the benefits.

By the 24-hour mark, your body switches to burning mostly fat. Inflammation drops, insulin sensitivity improves, and you enter full “deep repair” mode. As Dr. Vicki Catenacci says, “People now eat throughout a 14-hour window daily — fasting resets that.” With proper care, this natural reset may support long-term health.