There are a few reasons why Face ID may not work when you're lying down. First, your face is likely to be at a different angle than it is when you're standing up, which can make it harder for the Face ID sensor to accurately identify you. Additionally, if you're lying down with your head on a pillow or other surface, that can also block the sensor and prevent it from working properly. Finally, if you're wearing sunglasses or another type of eyewear, that can also interfere with Face ID. If you find that Face ID isn't working when you're lying down, try adjusting your position or taking off your glasses to see if that helps.