(CNN) Nearly half of the US population will see temperatures of at least 95 degrees over the next seven days, according to meteorologists.

More than 70 million people are under heat watches, warnings or advisories in different parts of the country, including the Midwest, the National Weather Service said Tuesday.

"Temperatures in the mid-90s to 100 degrees sounds bad, but the most significant aspect of the heat wave will be the overnight temperatures," CNN meteorologist Taylor Ward said. "Low temperatures from the Midwest to the Northeast will be in mid- to upper 70s. In some locations, the overnight temperatures may not drop below 80 degrees."

Ward said dozens of locations will set records for warm overnight low temperatures.

"Heat waves tend to bring the most severe impacts when the overnight lows are well above normal because there is no relief for people's bodies or homes to cool down," he said.

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