Most likely because as you are active over the course of the day it gets more inflamed and starts to give more symptoms by night time. Another possibility is that it is positional meaning that it hurts when you are laying down only, which you may not do during the day but experience when you lay down to sleep at night.

Two main reasons: one is the activity of the day, and the other is that there are less distractions. Pain at night that disrupts your sleep regularly should be checked by your doctor though.

Good luck,

Dr. Louden

Probably because you're lying there with nothing else to think about. It may be your mattress if it is too soft. Try doing Williams lumbar strengthening exercises. Google them. Do it every day. Your back pain will probably get better in a month or two.
Hope it helps!

Dr. Bose