While I fully believe in the benefits of power conditioning, I don't necessarily ascribe the difference in performance late at night to just power quality. There are other factors that can have a much greater impact. Rsbeck mentioned the psychological impact at night, and I think there is one even greater - reduced noise floor.
Almost everyone knows that a house is more quiet at night. The air conditioner runs less, TVs aren't on, water isn't running through pipes, less cars may be passing in front of the house, other people in the house are likely talking less, etc. If the noise floor in your house is lower late at night, then you absolutely SHOULD hear a difference in your system unless you listen in a sound-proof room. Use an SPL meter and take a reading in your room without your system playing both during the day and at night. If you find a difference, then it undoubtedly is one of the factors contributing to the perceived difference in performance.
Almost everyone knows that a house is more quiet at night. The air conditioner runs less, TVs aren't on, water isn't running through pipes, less cars may be passing in front of the house, other people in the house are likely talking less, etc. If the noise floor in your house is lower late at night, then you absolutely SHOULD hear a difference in your system unless you listen in a sound-proof room. Use an SPL meter and take a reading in your room without your system playing both during the day and at night. If you find a difference, then it undoubtedly is one of the factors contributing to the perceived difference in performance.