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.
The device has an on-board accelerometer. If the device sits still for 20 seconds, it goes into standby mode (lower heat setting, around 150C). Pick the device up or move it, and it will heat up to the last temperature setting selected. If the device sits still for 3 minutes, it will shut off. A bumpy car ride is enough motion to keep the device awake.