WASHINGTON -- The killer lurking in the shadows of the current heat wave may be hot nights.

"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."

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 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 National Oceanic and Atmospheric Administration reported at a briefing Thursday. And that's just all-time highs.

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.

When temperatures overnight do not drop enough, it increases the stress on people without air conditioning, on livestock and on crops, said Deke Arndt, chief of the Climate Modeling Branch at NOAA's National Climatic Data Center. In general, both daytime and nighttime temperature increases "are consistent with what we would expect in a greenhouse-warmed world," he added.