I would expect to get nearly full connection speed early in the morning. I'd say 7.8Mb is pretty good. If you have any large downloads to do, early morning is a time to do them.
I'm on a 20Mb connection but rarely get that speed. I might get around 18Mb at off-peak times. I tested the speed about an hour or so (09-30) after you did and got a download speed of around 11Mb and a upload speed of 683Kb. By 11-00, this had reduced to 5.5Mb download and 677Kb upload. It may get better at various times of the day (it is varying greatly either side of that right now). It does vary depending on the factors you've mentioned.
I don't find it particularly surprising that speeds go down at peak times. Many people's activity on Internet is quite bandwidth intensive these days. Film and music downloads and watching videos probably accounts for quite a bit. My ISP (Virgin Media) limits some people who upload/download large amounts of data between the peak hours of 16-00 to 21-00. This in order to keep up the speed at peak times for others.
It can be (and is) argued that ISP's should cope with all demands but I don't know what the practicalities are of doing so.
I'd think it's important to always use the same site for speed testing. Otherwise speeds might vary depending where the server is. I use this one:
http://www.broadbandspeedchecker.co.uk/