This question is a bit old but, since, a few tools have emerged which can help you choose when is the best time to trade. As this has been covered by other answers, I will not delve on the fact that Forex markets are open 24h a day except on weekends which means you can always resort to trading if you have trouble sleeping during the week
A friend of mine developed a very useful free tool available here, which will tell you which markets are open and their trading volumes at any time of the day. What makes it cool is that it will adapt to your timezone automatically so you don’t need to think about converting times when you use it at night or at any other time.
Please have a look (it’s a direct access, no registration needed or anything!) and don’t hesitate to leave some feedback there if you have any suggestions