Alfredo,
Another good reason to run in the morning is because I work from 7:15 AM to 6:00 PM and have a 45 minute commute to work every day. When I get home, I like to spend an hour or so with my 2 kids, one 18 months and the other 2 months, put them to bed at 8:00 or 8:15, and then relax with my wife. We tend to go to bed at 10 or 10:30, so I could go then, but I honestly think I would get a better run in at 5:00 AM then at 10:30 at night (when I then have to get up at 6:00 to get to work by 7:15 and start it all over again). The latter would actually result in about 6 hours of sleep (midnight to 6:00 AM) when the former would result in 7 (10:00-5:00). That extra hour also would probably help me as a runner.
So spend time with kids, wife and get an extra hour of sleep, but get up at 5:00 and sacrafice not feeling great on the run... OR, spend time with kids, wife and go for a very tired run at 10:30 PM and get only 6 hours of sleep.
I think the former wins.
Weekends are obviously not early morning runs... just Mon-Fri.