You don't want to give your lawn a close shave, or even a crewcut! Instead, keep its length at a few inches to ensure a healthy lawn. According to Gardeners, "To maintain a healthy root system that can resist heat and drought, do not mow your lawn shorter than about two inches. For even more drought resistance and to help shade out weed seeds, try setting your mower even higher, so your lawn is no less than 3 inches long."
Source: Flickr User Dan4th