You might also consider the difference in your ear's sensitivity at those extreme times of the day. Your ears will naturally become less sensitive when the environment is noisy (loud). And then at night, or early morning, they can pick up the slightest details. Try wearing ear plugs for an hour or so, and then take them out to see what I mean.
Different & inconsistent sound quality
I am noticing different sound quality from my audio system, depending on when I listen to them. My system consists of Krell integrated amp, B&W CDM 1SE, and Sony DVP-7700. They are connected with Transparent ML Super interconnect and MIT T2 Biwire speaker cable. For example, in the moring and very late night, piano recordings sound very clean, detailed, and transparent, however, in late afternoon or evening, it seems that some of piano notes from the same CD sound very dull and veiled. The phenomenon is true of other recordings of cello, violin, and etc. I suspect that AC noise has to do with the problem, so as a first step, I consider upgrading stock power cord for my integrated amp to a better one such as Transparent PowerLink Super or others. Could anyone help me with any possible solutions to the problem I'm experiencing? Any suggestions or comments would be greatly appreciated.