Although we allow you to hold positions overnight, please note the following:
Every day, the Exchange sets a settlement price in order to calculate each trader's PnL for that day at market closing. If you leave a position open overnight, then your PnL for the 2nd day will start calculating the settlement price towards the current price, instead of where you initially opened the position. This does not mean you lose any profits/losses generated on the 1st day as the total equity is the same, the only thing that is different is your PnL for the day. You can read more on CME article here.
Example:
John buys 1 ES contract on Monday morning at 1925.00 and sells the contract on Tuesday morning at 1930.00.
John's overall profit for this trade is 250$ minus commissions.
However, if the Settlement price on Monday's market close was set at 1927.50, then John will see on Tuesday that his PnL is +$125, instead of +$250, as his PnL for Tuesday started counting from the settlement price (1927.50) up to his exit price (1930.00).