It depends on the contract, but I have not seen one that is open 24 hours a day on any exchange. More like 23.5. Also, you can only trade them if there is someone to take the other side. But there usually is.
The ES are closed from 4:15 pm - 4:45 pm Eastern (someone correct me if I'm wrong) for no good reason. I'm sure the CME has some bs reason, maybe they are running MS Windows, which according to Bill Gates should be rebooted at least once per day.
Gord, If you really can't sleep at night, you may want to look at the 10 yr bond on ACE. The volume is usually higher than the mini Spoo/Naz, and it picks up around 2:30 AM - 3:00 AM (NY time), that's when the London market opens. Beware, it's not the same game, spreads are usually larger than day time US. Can't sleep? maybe you better go to a casino, you may have more fun, free drinks and pretty women (not free!)