NEW YORK: American TV viewers were exposed to an average of more than one hour of TV ads each day in 2013 and the proportion of ads aired during each hour of programming has steadily risen over the past five years, a new study has shown.

The latest "Advertising and Audiences" report from research firm Nielsen disclosed that commercials accounted for 14 minutes and 15 seconds of each hour's worth of network TV programming in 2013, up 50 seconds since 2009.

Ads on cable TV secured even higher rates, the study found, averaging 15 minutes and 38 seconds during each hour of programming last year, although this was down from the 16 minutes and 11 minutes recorded in 2011.

With Americans estimated to watch about five hours of TV each day, the report calculated that advertising accounted for about one-quarter of the total – or more than one hour's exposure a day – Marketing Charts reported.

In addition to the increased broadcast time allocated to ads, Nielsen also identified another trend for ads to be shorter in length.

It found TV ads shortened to 15-seconds accounted for 44% of all commercials aired in 2013, up from 35% since 2000, while the more traditional 30-second spot decreased from 62% to 53% over the same period.

TV ads lasting 60-seconds continued to comprise 2% of the total last year, the same proportion as in 2000, although this was down from 5% in 2005.

The report attributed the growing popularity of 15-second spots to the "changing video ecosystem" and observed that 95% of "neuro-optimised" shortened ads "performed as well or better than their original :30 counterparts".

The demographic profile of audiences is also evolving, Nielsen said after studying primetime TV usage among different ethnic groups from 1995 to 2014.

According to the report, the nation's African-American households increased in number by almost 40% over that period to 14.9m. The figure for Hispanics more than doubled to 14.7m and Asian-Americans made up 5.2m of TV households.

Data sourced from Nielsen, Marketing Charts; additional content by Warc staff