This will totally depend on your strategy and instrument of choice.
When a market opens or closes there can be more volatility and there is better volatility when several markets are open at the same time. Having said that during Asian hours there can be some interesting movements and you might just get in really nice and early to a big move at London open.
You just have to try different ideas by putting time filters in your strategy. Beware of over fitting the time element though. There could have been one massive win between 0900 and 1000 that makes your strategy look great but without that one trade the strategy is average or even rubbish.
I am not a fan of closing a trade just because the clock has reached a certain time. It can be a terrible way to curve fit. I prefer to pay the overnight fees and let the trade have some room to develop.
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