Are you sure? here is says it starts on 8pm for both nasdaq and nyse http://dynamic.nasdaq.com/dynamic/afterhourmanyse.stm my T&S wasnt printing anything before 8pm also(except on spy)
Not becuase of regulations obviously... I found this (probably a former ECN): ================ NexTrade www.nextrade.org Market Maker ID: NTRD Developed in the late 1990s and owned by Professional Investment Management, Inc., NexTrade was the first ECN to offer 24-hour trading to retail investors through participating broker-dealers. Its clients include mutual funds, hedge funds, market-makers, traditional broker-dealers, online broker-dealers and professional trading companies. In 1999, NexTrade officially applied with the SEC to become a registered, fully-electronic, 24-hour securities exchange. Additionally, NexTrade has a pending application to become a fully-electronic futures exchange. NexTrade Hours of Operation Sunday through Saturday (including holidays) 24 Hours-a-day 5:15 p.m. to 9:00 a.m. Eastern Time - matching session for NYSE listed stocks.