What is the drowsiness test? The convenient thing about breathalyzers is it provides a quantitative number we can use in judgement. I can't think of a similar

What is the drowsiness test? The convenient thing about breathalyzers is it provides a quantitative number we can use in judgement. I can't think of a similar test for drowsiness. Maybe there is a blood test? Maybe someone could create a similar device that looks for special metabolites in the breath to provide a measurement for drowsy people?