In 1965 , LA had 3 big Top 40 stations known as KHJ, KRLA, and KFWB. All of them had 3 hour block shifts except overnights. WABC in NYC had a two hour shift from 10-Midnight. Eventually, shifts around the country were changed to a fairly universal 6-10, 10-3, 3-7, 7-12, and 12-6. So why not 6 hour shifts? WPOI in Tampa has six hour shifts now. Dick Ring on WDUV in Tampa does a 6-10 morning gig and reads the weather until about 2PM as they have only one live shift. COX radio in Florida runs most of its stations on primarily auto pilot. They don't even voice track. And they seem to succeed with this. You can always do Delilah and Tesh back to back like Lite 98 in Richmond Va. Once they were live 24 /7------but adding these "personalities" has not changed their market share.