plga-
What you are hearing is perfectly normal, and there is not a thing you can do about it. For the simple fact that anything you do to make it better in the daytime will also make it even better at night. There really is no way around it.
If you are really trying to recreate your night time experience in the day time, the closest you can get is to flip off all the breakers. Don't merely unplug appliances. That helps but leaves all the wire in the walls still gathering up RFI like the great big antennae that they are and feeding it all right into your system. Flipping breakers physically disconnects a lot of that, with the resulting lower noise floor closely resembling what naturally happens late at night.
Or you could get the Synergistic Research FEQ. And only run it in the daytime. Never at night. Otherwise.... see 2nd sentence above.
Congratulations btw on noticing. Lotta guys never do figure this one out. Plenty more won't believe it even when you tell them. Audiophiles! Ha! Since you do notice however one thing you better not ever try, waiting up till 2am, flipping off the breakers, running a demagnetizer through all your cables, spraying (ordinary laundry) anti-static spray, demagnetizing your record, and then listening.
You think you got problems now? Just don't do it.