Hello and welcome, Thanks for your query. When you say your pain is usually at night, is it because you sleep on your back and thus apply pressure on your lower back and/or left leg (mainly till your mid-thigh) ? Does the pain start from her lower back and go down till mid-thigh? If yes, this could most probably be her Sciatic nerve being irritated and is also called Sciatica. Let me list few of the most common causes of sciatica: -Degenerative disc disease: The disc s in every human being undergo a degenerative process with age, and is considered to be normal. When these disc s undergo degeneration they tend to irritate the nerve root present in the lower back, which can result in sciatica. -Sacro-iliac join dysfunction: the lowest end of your spine consists of fused bones called the Sacrum. This bone articulates with the iliac bone (one of the three bones forming your pelvis), thus this joint is called the sacro-iliac joint. Any irritation of this joint can also cause the same pain that would be typical for sciatica. -Piriformis syndrome: There is a muscle present in the buttock region called the Piriformis muscle. The sciatic nerve passes under this muscle, and any irritation of this muscle can also lead the sciatica. -Herniated disc: The Lumbar part of your disc can develop a hernia which is when the inner core of the disc leaks out or herniates. This protrusion can irritate the nerve roots, also resulting in sciatica. --Isthmic spondylolisthesis: In this condition a small fracture allows one vertebral body to slip forward on another. Due to this the sciatic nerve can get pinched and cause the pain. -Lumbar spinal stenosis: Stenosis (narrowing) of the spinal canal can cause the sciatic nerve to get pinched and cause this pain Other causes can be: muscle strain, spinal tumors, pregnancy, scar tissue, infection, direct injury, etc. Please consult your doctor on the same and try to find the cause in order to properly manage the sciatica as different cause require different management techniques. I hope I have succeeded in answering your questions, you could write back to us anytime for further clarifications. Best wishes.