After your FUE hair transplant, it is very important to rest for the first three days. When sleeping, sleep on your back, never sleep on your side, or you should not sleep face down. If you sleep face down, swelling might occur on your face, if you sleep on your side, your eyes might get swollen. Therefore, it is best to sleep on your back, and this is comfortable for you.
During this stage, you can apply ice on your forehead or your eyes, and the tissues will be cold which will prevent swelling. While resting during the first three days, reading a book, playing on your phone, using your computer and such might cause swelling. Therefore, it is best to rest on your back and sleep on your back for the first three days. It is for your comfort.