All I can do is share my experience with you. I know for a fact... that my system sounds better in the morning than it does at night. And on my days off, when I listen throughout the day...It varies when it sounds better and worse. "I" think it has to do with the AC lines and what's going on in the neighborhood and with my neighbors at the time. Think about it...it's the only thing that makes sense. We all share the same power. At night...more people are using appliances that suck power, and it puts a sag in the shared power supply. You need to check out a product by PS Audio called the Premiere Power Conditioner. Go to this link from audio advisor. Go to the video tab and check out this video from PS Audio.
http://www.audioadvisor.com/prodinfo.asp?number=PSPREMIER
He goes into how the unit "regenerates" crappy AC coming into your home, and converts it to pure, clean AC. Then...it has a voltage regulator to ensure your system is getting a constant 120 volts to all the components, all the time, instead of dips and sags.