This is a methodological paper with the aim to discuss pros and cons related to different tools and environments when evaluating the effect of warnings given to sleepy drivers. There is no simple answer to the question which platform is most suitable. It depends on the research question asked, and it is possible that different aspects of the problem should be approached with different methods. A driving simulator has clear advantages when high control and repeatability are paramount. A simulator can also be used when the driver has to be put into a potentially dangerous scenario. How ecologically valid the results obtained from a simulator in fact are depends very much on the fidelity of the simulator. A test track study is based on real driving and should have a higher degree of ecological validity. On the other hand, the test track most often consists of an unrealistic environment. For assessing the prevalence of drowsy driving in real traffic, and in order to investigate what drivers actually do when they receive a sleepiness warning, it is absolutely necessary to study their natural behaviour when they go about their daily routines. Here field operational tests or naturalistic driving studies are most suitable. A disadvantage is the lack of control.