Did you have an MRI of your lumbar spine to get your diagnosis of DDD and sciatica? If you haven't I suggest it if for no other reason then ruling it out as the cause. It sounds to me like your sciatica could be triggering daytime symptoms. When I had a sciatica issue years ago I had trouble sitting. Perhaps that's why your daytime symptoms are worse then nighttime which is opposite to many of us.

My symptoms are worse when I sit also and I don't have sciatica problems now. Certain chairs help lessen it for me. A back support for the chair might help.

I also have DDD along with many other spinal issues. I am seeing a chiropractor who is helping. My spine is not the cause of my symptoms but it is a contributor. It's a good thing to know.

if it is only affecting your left leg, it could be compression issue, or your lower spine problems. HAVE YOU had scans in that area.