According to Nielson DVRs are increasing the amount of time people spend watching TV. Nielson compared total TV usage for 18-49 year olds between November 2005 and November 2007; the data from 2005 was before wide spread adoption of DVRs and prior to Nielsen measuring DVR homes for its numbers.
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The data shows that viewing had increased slightly throughout the day and was a full 3% higher at 9pm and 5% higher between 11pm and midnight.
Nielson says that the prime time period between 8pm and 11pm was expanding because people are watching shows they recorded alter in the evening. I can certainly see this; I rarely ever watch a live TV program any more. The vast majority of TV my family watches are programs we recorded at different times or on different days.