Why only trade 9 -12:00am?

"Somewhere on the planet, financial centers are open for business, and banks and other institutions are trading the dollar and other currencies, every hour of the day and night, aside from possible minor gaps on weekends. In financial centers around the world, business hours overlap; as some centers close, others open and begin to trade. The foreign exchange market follows the sun around the earth.

The International Date Line is located in the western Pacific, and each business day arrives first in the Asia-Pacific financial centers— first Wellington, New Zealand, then Sydney, Australia, followed by Tokyo, Hong Kong, and Singapore. A few hours later, while markets remain active in those Asian centers, trading begins in Bahrain and elsewhere in the Middle East. Later still, when it is late in the business day in Tokyo, markets in Europe open for business. Subsequently, when it is early afternoon in Europe, trading in New York and other U.S. centers starts (see diagram). Finally, completing the circle, when it is mid- or late-afternoon in the United States, the next day has arrived in the Asia-Pacific area, the first markets there have opened, and the process begins again." – Sam Cross, Federal Reserve Bank NY.

The Foreign Exchange market is broken up into three major trading sessions: the Asian Session, the European Session, and the U.S. Session. If you live in London, trading starts at 07:00 and finishes at 17:00. Then the US Session opens at 12:00 and closes at 22:00 when the Tokyo session opens at 22:00 and closes at 09:00. However, these times are approximate and the sessions overlap as one session leads into another. The forex market experiences greatest liquidity during these points of overlap as more participants use the market than at other times.

The STAR:fx model trades the European Session because the high degree of overlap with other Sessions means higher liquidity which produces more competitive pricing and tighter spreads, it participates in the day's major currency moves and we stop before the more volatile US Session begins.