it’s depend what strategy you use, and pair you trade, 
hooray my first post
Are you saying that there is nothing like time frame?
It has to be day. Why trade at night when the markets are dead. You can’t make money there
I chose the night, because the atmosphere is more quiet to think and analyze the markets.
You can choose time best for you according to your time zone and trading session what suits you. I do full time trading on demo and try trading in different timings of day and night .It make me clear the market movement that is good for me for my trading style.
My experience shows that the worst time would be when the market opens that’s when the market volatility is very high, in other words its best to check the market for times where its stable and effected by other markets opening. its similar to other financial markets where there is an arbitrage which effects the stocks price.