Arduino SleepMode without interrupts

I am doing some project with ultrasonic sensor HC-SR04, and I need to put Arduino to sleep for 30 seconds,but i want to do my code without interrupts. Can i get some help please?

Impossible, only an interrupt of some sort can wake it up…

But there are some libraries out there that do all the stuff for you. Btw, the Arduino will not sleep for 30 seconds straight, longest the Arduino can sleep is 4 seconds. But by putting it back to sleep right away the overhead is minimal.