That's because Alanna suffers from the extremely rare Kleine-Levin Syndrome (KLS), a brain disease that causes "episodes" during which she exhibits toddler-like behavior, including throwing fits, talking in a baby voice, sleeping extensively, binging on junk food, and forgetting how to read.
Alanna has no memory of her episodes once she comes out of them. While some episodes may last only a couple of days, others can be much longer, with Alanna's longest one lasting eight months. She claims to have lost six years of her life to the condition, which she was diagnosed with at the age of 10.
For more of Alanna's story, check out this lengthy profile of her on The Sun's website.