Trading in overnight stock futures has skyrocketed, adding to a nearly non-stop stretch of market activity and luring more investors to join in the action.

Among the most traded are E-mini S&P 500 futures contracts, whose overnight trading volumes have surged to a record this year, according to CME Group data through March. Daily average volumes have topped 500,000 contracts, more than double the number recorded in 2017. The overnight session begins at 6pm Eastern Time on Sundays and weeknights, and ends the following day at 9am.