Leather is a natural material. However, it can be sensitive to sunlight, so it's essential to take steps to protect it. Leather exposed to the sun for long periods will start to fade and crack. It will also become more brittle and easily damaged.
Sunlight contains ultraviolet rays, which can break down the oils in the leather, causing it to dry out and crack. This is why your leather bag mustn't stay in direct sunlight for extended periods.