The question was, "Question raised in PT yesterday, re total knee & ACL reconstruct. Why does it hurt most or only at night?" This person was experiencing a very common complaint after knee surgery (or other surgeries, or other problems that cause pain--such as arthritis). The short answer is that for any given patient, there might be a variety of reasons that night pain is greater. The most common reasons, however, are two-fold:
First, during the day there are significant distractions. The lights are on, there is noise, and stimulation from a variety of sources for most people. At night, when the lights go off, the amount of distracting stimulation goes to a minimum--and the painful stimulation of an arthritic, or postoperative area of the body takes center stage. Focusing on the pain elevates the perception of pain.
The second reason pain might be greater at night is that it is the first time the the joint has been rested all day, and significant swelling may have occurred. Most of our mesenchymal tissues (parts of the body that form the musculo-skeletal system) undergo activity related microtrauma with exercise and require rest to recover and return stronger. Physical therapy is therapeutic exercise and is usually directed towards muscles that have become atrophied or injured...and it is normal for those tissues to ache after exercise.
Hope this explanation is helpful!
Dr. Jeffrey Lyman