Since currency pairs are more active in relation to different sessions, you should tailor your strategy to fit its most favorable session. If your strategy were based on capturing short-term trends and breakouts on the major currencies, you might want to trade only the London and New York sessions, or their overlap, where there are more active traders and bigger, more consistent price movements. If your strategy were based on scalping and counter-trends, with the minors or yen crosses, you might want to place your focus on the more Asian session. If you are trading without a robot or EA, you must also consider your physical well-being: a market participant from the U.S who has a day job would have difficulty trading the active hours of GBP/JPY during the Asian session, and a better alternative would be to trade the European/U.S session overlap, where volatility is still elevated enough to trade GBP/JPY even though the Asian session is closed.