millions. to see just how that works we went to the jefferson sleep disorder center. i had my vitals taken. 98.1. reporter: and went outside to cook. 134 degrees. i m sweating. let s go back in. 98.4. reporter: i m fitted with electrodes. people when they re exposed to a lot of heat, when it s really hot outside, they tend to wake up more, they tend to arouse. reporter: that s because when the body sleeps air core temperatures have to cool down. if you re overheated your body won t cool and your sleep is disturbed. cooling off hasn t been easy. here in philadelphia they crushed into municipal pools by the thousands. matt gutman, abc news. now the warning signs for heat-related illnesses. they re not too difficult to see. as matt mentioned insomnia can be a problem during a heat wave like this one. abc s medical editor dr. richard besser has some guidance. this is really important. listen to this. in 10 to 15 minutes if you ve stopped sweating your body temperature can g