In the testing literature, the term load testing is usually defined as the process of exercising the system under test by feeding it the largest tasks it can operate with. Load testing is sometimes called volume testing, or longevity/endurance testing.
Keep your taste buds awake, drink an energy drink. This may raise your level of alertness for a reasonable amount of driving time. Suck or munch on an apple, orange, or even a lemon to keep taste buds awake. The more tangy the food item the better. (Pull over somewhere safe before eating anything that you can't just pop in your mouth, however.
Here's what you need to do: Hold down the sleep/wake button (on older models it's on the top of the phone. On the iPhone 6 series and newer, it's on the right side) until the power-off slider appears on the screen. Let go of the sleep/wake button. Move the power-off slider from left to right.