rules of the exchange used to be decided by brokers who own the exchange. the brokers are now the customers and the exchange is a business. with smart phones people can make trades in lunch break. it was the reason for premarket and afterhours so people who work full time can enter trades. there is no for overnight as not enough volume the fact you even suggest shows the exchange want to have access to foreign traders in asia or other time zones for 'trading' it will have no effect on the company who shares are listed with 24 hour trading since most trading automated. will 24 hour trading increase cost of brokers? probably not. there is no volume even afterhours and pre market so what is the point of 24 hours?