In addition to making sure that the testing conditions are standardized, it is also essential to ensure that your pool of participants is the same. If the individuals in your control group (those who are not sleep deprived) all happen to be amateur race car drivers while your experimental group (those that are sleep deprived) are all people who just recently earned their drivers licenses, your experiment will lack standardization.
When choosing subjects, there are a number of different techniques you can use. A simple random sample involves randomly selecting a number of participants from a group. A stratified random sample requires randomly selecting participants from different subsets of the population. These subsets might include characteristics such as geographic location, age, sex, race, or socioeconomic status.