My daughter is 6 months old and she's been doing this since about 3 months after coming home from being in the hospital with RSV and pneumonia. With this she was on many medications to stay sedated. Morphine included. I can't even count the amount of medications that were being pumped into her little body. I'm worried that's why she shakes her head back and fourth quickly. She only does it when she's laying down and a lot when she's sleeping. And she's only been doing it for the last three months. doctor tells me it's normal.... She shakes her head so quickly I'm afraid she's going to hurt herself.