When the sun goes down during a heat wave, there's more to worry about than finding the cool spot on the pillow. Real danger may be just beginning.

When the sun goes down during a heat wave, there’s more to worry about than finding the cool spot on the pillow. Real danger may be just beginning.

For folks without air conditioning, a nighttime respite from the worst of the heat gives the body a vital chance to recover from the stresses of the day. But as searing heat moved into the Northeast and its big cities on Thursday, experts cautioned that those stresses can actually rise when nighttime air doesn’t cool to levels that provide relief.

“Everybody kind of gets fixated on how hot it gets: `Did we break 100?'” observed Illinois state climatologist Jim Angel. “But the nighttime temperatures can be just as important.”

Those trying to find ways to beat the nighttime heat have to get creative.

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“I jump in the pool or take a shower and go to bed with wet hair,” said Joan Peltier, of Toledo, Ohio, where temperatures soared past 100 on Thursday and the heat index around 10 p.m. was still 95 degrees. Her husband sometimes sleeps wearing a wet T-shirt.

The National Weather Service has issued excessive heat warnings for a huge section of the country, from Kansas to Massachusetts, forecasting temperatures near or at triple digits Friday and into the weekend.

The high temperatures and smothering humidity will force up the heat indexes, as well. Boston’s 99 degrees on Friday could feel like 105 degrees, Philadelphia’s 102 degrees like 114 degrees and Washington, D.C.’s 103 degrees may seem the same as a melting 116 degrees. Nighttime low temperatures could range only from the mid-70s to the low 80s.

In New York, people looking to beat the heat were thwarted by warnings urging them to avoid city waterways after a wastewater treatment plant disabled by fire began spewing millions of gallons of raw sewage into the Hudson River.

Officials cautioned against swimming and bathing at Staten Island’s South Beach, Midland Beach and Cedar Grove Beach, and Brooklyn’s Sea Gate beach, especially for people with medical conditions.

Across the country, emergency room visits were way up, according to public health officials, mainly because of people suffering from heat exhaustion and heat stroke.

The weather is suspected of contributing to a number of deaths nationwide. At least six more fatalities were reported Thursday, including a Michigan restaurant cook who suffered a heart attack after being sent home from his job and a teenage boy who drowned while swimming at summer camp in the same state.

It wasn’t clear how many of those deaths occurred at nighttime, but experts note that the dangers can’t be ignored.

High overnight readings increase energy consumption as air conditioning units run deeper into the night and start earlier in the morning, said Deke Arndt, chief of the Climate Modeling Branch at the National Oceanic and Atmospheric Administration’s National Climatic Data Center.

That problem was a major factor in the 1995 heat wave that struck Chicago, claiming more than 700 lives, Angel pointed out in a telephone interview. There were reports of elderly people who died in apartments because they couldn’t afford to run their air conditioning or were afraid to open windows because of the fear of crime, he noted.

Chicago has since improved its response to heat waves, adding more cooling centers, sending people out to check on the elderly and asking neighbors to assist one another, he said.

A blown electrical transformer in the Detroit suburb of Ferndale forced several senior citizens to sleep Wednesday night in the community room of their six-story apartment building after the power failed.

On Thursday morning, Lisa Blumentritt ventured back into her third-floor unit.

“You couldn’t breathe,” she said at a nearby cooling center. “It’s like 100 outside and 110 degrees inside. I couldn’t go back there. This is extreme.”

While the current heat wave has recorded 12 all-time daily highs so far this month, it also has registered 98 all-time overnight highs, the NOAA reported at a briefing Thursday.

When it comes to a record high for a particular date, 1,279 locations have tied or broken daytime records this month while 3,128 nighttime highs have been tied or broken.

For example, on Wednesday Eppley Airfield in Omaha, Neb., had an overnight low of 82 degrees. That was a full 5 degrees warmer than the previous warmest overnight on that date, set in 2002.

Litchfield, Minn., also posted an overnight low of 82, besting a warm nighttime record for that date of 74 degrees set in 1964, and the overnight low of 82 at Lambert Field in St. Louis edged out the 1998 mark by 1 degree.

Cities are worse for heat deaths because they remain hotter at night from the “heat island effect,” in which urban infrastructure absorbs heat and let go of it slowly, experts say. And older northern cities – where many homes lack air conditioning – are worse off than Sunbelt cities where most people have cooling.

Associated Press writers Randolph E. Schmid in Washington, John Seewer in Toledo and Corey Williams in Ferndale contributed to this report.