I don't see any difference no matter what time of the day I have entered. For the most part, I enter during regular daytime hours and hit a lot on the instant sweepstakes. Since we have moved a couple of months ago, I was doing a good chunk of them late at night and wasn't even hitting as much as I have during the day.
You can also say that most people are at work during the day, so there are less people sweeping during the day also. Sweepers that work enter their sweeps at night into the wee hours. Then, there are people who work odd shifts entering at odd hours.
I've entered sweepstakes at all times of the day in the last 13 years. Personally, overall, I don't see any particular time of entering being better than another. That's my story, and I'm sticking to it!