Measuring sleepiness is difficult. In the sleep lab, we measure it by timing how quickly someone can fall asleep when attempting to nap. Sleepiness can affect our ability to drive, operate other heavy machinery, and perform complex mental tasks. The sleep field is working on a way to measure sleepiness and its affect on performance. If that could be done relatively easily, then we could determine who might be dangerous on the road, for example. Researchers are looking for the equivalent of the breathalyzer for sleepiness / performance impairment.
Researchers have noticed that the variability in heart rate (HRV) is associated with sleepiness. In the March 2012 Journal Sleep, Dr. Chua and colleagues studied the HRV in sleep-deprived subjects and compared it to a standard measure of mental performance. HRV is determined by a computer program that analyzes the beat-to-beat changes in the heart as measured by an EKG. Subjects were kept awake for 40 continuous hours and the study results showed that HRV correlated with measurements of mental performance.
The authors concluded that EKG-derived measures could possibly be used to determine a person's sleepiness level. However, one problem with measuring sleepiness in this way is that it does not determine absolute level of sleepiness. You would have to determine the normal HRV in each person when they were completely rested before you measure it when they might be sleep deprived. Also, it is not known whether the HRV can determine sleepiness on a moment-by-moment basis. Finally, other factors can affect HRV, like cardiovascular disease and certain medications. Therefore, more research will be needed before HRV can be successfully used to measure sleep-deprived mental performance.