The study author said that those twin trends suggest a growing number of people are "spiritual but not religious."
The report found that in addition to an increased number of people who pray, a growing number believe in the afterlife. When asked how they view God, the most common responses were the traditional images of father and judge.
Sociologists of religion say the rise in people who are spiritual but religiously uncommitted is prompting churches to repackage their services into more contemporary offerings with fresh, livelier music and less of the usual liturgies.
"Americans' attitudes toward religion are growing more complex. While fewer people identify with a particular religion, belief in God remains high," said study author Tom W. Smith, Director of the General Social Survey at the National Opinion Research Center at the University of Chicago.
"When asked simply about belief in God, most people include a range of God images, from a personal God to believing in a 'higher power' or a 'spirit or life force,'" he added.
The study found that people who don't believe in a personal God but in a higher power of some kind rose from 5 percent in 1964 to 9 to 10 percent in recent surveys.
The report is based on data from the General Social Survey, the nation's longest, most scientifically reliable source of information on American attitudes and behaviours.
In the U.S., belief in God has ebbed over time from about 99 percent in the 1950s to about 92 percent at present. Certitude about God also has diminished, but the vast majority of Americans still express a strong and close connection to God.
The GSS has asked people for their images of God since 1984, and about half of the people have consistently referred to God as "father," while others used terms like "master" or "judge" to describe their idea of God. The number reporting God as "mother" has stayed at about 3 percent.
Although belief in God remains strong, the survey found that 22 percent of people said they had never attended a religious service, compared with 9 percent in 1972.
The trends toward reduced church attendance began in the mid-1980s, and by the mid-1990s, fewer people reported identifying with a particular religion.
In the most recent survey, 16 percent of people reported "none" when asked about their religious preference, a figure that stood at 5 to 8 percent in surveys taken between 1972 and 1991.
Daily prayer rose from 52 percent in the 1989-90 survey to 59 percent in the most recent survey. Belief in the afterlife also went up modestly, from 69 percent in 1973 to 73 percent in most recent surveys.