It can be a great way to beat the heat and I especially like to ride an hour or so in the dark before sunrise. I am usually on very empty rural roads when I do this and use a rear blinkie and a headlamp, although sometimes I don't bother with the headlamp depending on road conditions. I do typically also have on some reflectorized clothing and believe that I am more visible than in daylight on dark rural roads. At that hour it is so quiet that I can usually hear any approaching traffic a mile off.
In a busy town with lots of traffic and confusing lights I am more cautious, but I really think that on dark rural roads riding in the dark is safer than riding in daylight.