I ve had knee pain practically from the time I was born, 16 now. Female. And it only comes on around night. Its kind of a mild pain, but lasts for extended periods of time. It gets to the point where I cannot sleep. I ve been told its growing pains, but its located behind the knee cap. I m not very active, so its not an injury. The only way I ve found relief is keeping my leg straight, but this can be difficult because it makes it hurt worse for the time being. If I bend my leg it offers little relief now but just makes it worse later on.