No relief from the heat as many US cities will see record overnight temperatures

Health experts say overnight temperatures that fail to cool down are even more dangerous than daytime temperatures that soar.

It has already been a sweltering start to the summer across much of the U.S. due to the long-lasting heat dome expected to blanket much of the country this week. The blistering temperatures over the past few weeks have caused heat-related deaths in New Jersey and helped fuel wildfires in the West.

No relief from the heat at night this week

Temperatures were not forecast to drop below 80 F (27 C) at night in Fort Lauderdale, Florida; Miami; Tampa, Florida; Galveston, Texas; and Charleston, South Carolina, the National Weather Service said.

Over the next few days, places in the Midwest and Northeast known for frigid winters will see nighttime temperatures remain above 70 F (21 C), including Fargo, North Dakota; International Falls, Minnesota; and Portland, Maine.

Hot temperatures at night pose a bigger danger

Health experts say that high overnight temperatures are particularly dangerous because there's no time for the body's core temperature to cool down and recover from daytime heat.

“That’s where the health outcomes are amplified, particularly for the elderly and vulnerable communities,” said University of Georgia meteorology professor Marshall Shepherd.

Just a few degrees of increased body temperature can lead to heatstroke or put too much strain on the heart.

Dangers of heat can sneak up on you

If temperatures at night don't cool down your body, the health risks more often show up the next day, said Kristie Ebi, a public health and climate scientist at the University of Washington.

“Mortality starts the second or third day" because the body's unable to cool, she said on Monday.

Being proactive is crucial during a heat wave

Early warning signs include heavy sweating, muscle cramps and headache. “It’s hard to know you’re getting in trouble with the heat. This is why we need to be more proactive," Ebi said.

It's important to find a way to cool off, whether stepping into air conditioning or wrapping a cold towel around your neck.

And health experts say don't forget to check in on friends and family members, especially those who are older, pregnant or who have health challenges that might make it more difficult to handle the heat.

How to beat the heat without air conditioning

Even without air conditioning at home, there are ways to find relief.

Stop by a library or a mall. Sit in front of a fan and spray water on your skin. Put your feet in cold water. Soak your clothes in water. And of course, drink plenty of water.

Associated Press reporter Seth Borenstein in Washington contributed.