During heat waves, like the one that now has a grip on much of the East, one of the major causes of heat deaths is the lack of night cooling that would normally allow a stressed body to recover, scientists say.
Some scientists say the trend is a sign of manmade global warming.
A top federal research meteorologist said he "almost fell out of my chair" when he looked over U.S. night minimum temperature records over the past 96 years and saw the skyrocketing trend of hot summer nights.
From 2001 to 2005, on average nearly 30 percent of the nation had "much above normal" average summertime minimum temperatures, according to the National Climatic Data in Asheville, N.C.
By definition, "much above normal" means low temperatures that are in the highest 10 percent on record. On any given year about 10 percent of the country should have "much above normal" summer-night lows.
Yet in both 2005 and 2003, 36 percent of the nation had much above normal summer minimums. In 2002 it was 37 percent. While the highest-ever figure was in the middle of America's brutal Dust Bowl, when 41 percent of the nation had much above normal summer-night temperatures, the rolling five-year average of 2001-05 is a record, by far.
Figures from this year's sweltering summer have not been tabulated yet, but they are expected to be just as high as recent years.
And it is not just the last five years. Each of the past eight years has been far above the normal 10 percent. During the past decade, 23 percent of the nation has had hot summer nights. During the past 15 years, that average has been 20 percent. By comparison, from 1964 to 1968 only 2 percent of the country on average had abnormally hot nights.
"This is unbelievable," said National Climatic Data Center research meteorologist Richard Heim. "Something strange has happened in the last 10 to 15 years on the minimums."
But it is not surprising because climate models, used to forecast global warming, have been predicting this trend for more than 20 years, said Jerry Mahlman, a climate scientist at National Center for Atmospheric Research and a top federal climate modeler.
It is a telltale sign of global warming, Mahlman said: "The smoking gun is still smoking; it's not shooting people yet."
One reason global warming is suspected in summer-night temperatures is that daytime air pollution slightly counteracts warming but is not as prevalent at night, said Bill Chameides, a climate scientist for the advocacy group Environmental Defense.
The records for summer-night low temperatures are part of a U.S. Climate Extremes Index developed by the National Climatic Data Center. Last year, in large part because of record hurricane activity, saw the most extreme weather in the United States since 1910.