They are not the same thing
Sleep deficiency affects different people in different ways. Some get hit hard and measures of performance are markedly lower, while other people can brush off some deprivation with little outward effects. You can’t look at how much a person has slept recently and necessarily predict how the person will suffer. The underlying brain physiology explaining this difference has not been discovered.
Sleepiness, like sleep debt, results in lower performance on reaction tests. Sleepy people are not as vigilant, not as mentally quick. This could be due to a decline in brain activity in the prefrontal cortex – the part of the brain where higher order functions tend to be located. The evolutionarily older parts of the brain are maintained at the preference of the prefrontal cortex.
Lower activity (either by electrical measures, glucose consumption, or blood flow) in the pre-frontal cortex could possibly be a biomarker for sleepiness. But it would not necessarily be a biomarker for reduced aptitude or performance.
Reaction time is one way to measure sleepiness, but it is not a very good way. It can be affected by many things (e.g. medication, mild brain injury) so it lacks “specitivity” in the parlance of experimental design.
Diagnosis of sleep disorders and other mental phenomena often incorporates tests. Some people are affected more by mild sleep deprivation than others, as measured by tests of vigilance and mental and physical performance. The rate of degradation is consistent for individuals, which suggests a genetic phenotype. The details are unknown, but it is probably something at least partially genetic like chronotypes are.
Sleep loss can cause some sections of the brain to partially “shut down” – or more precisely be less active as measured by imaging technologies that can show blood flow and sugar consumption. However, the plasticity of the brain allows other portions of the brain to step up during these periods of fatigue and mitigate the performance degradation as observed by behavioral tests.
Identifying Sleepy People
It would be great if we could find reliable markers of sleepiness and of vulnerability to performance decline in cases of extended waking. The military, for instance, would love to know which of its members can perform in extreme situations, as would operators of airlines and trucking lines.
It is not known if people who experience a large performance decline in total sleep deprivation are also more susceptible to a decline due to chronic sleep deprivation (partial sleep deprivation). One study tried to find out if the rate of sleep debt accumulation produced different results in behavior tests but it was inconclusive.
Chemical biomarkers for sleepiness are notoriously lacking. You can administer tests to determine vigilance or readiness, but there is no way to say how an individual will perform in a future state of sleep deprivation. Scientists have identified a gene (the human leukocyte antigen (HLA) DQB1*0602) that might be a candidate for figuring out who is vulnerable to sleep deprivation. No tests have yet been developed to take advantage of this finding.
Scientists strongly suspect a genetic component in variation among individuals. Train all you want, but some people can better handle sleep deprivation than others. By “handle” we mean perform mental and physical tasks when sleepy as well or close to as well as when rested.
The ideal behavioral biomarker would indicate a change in some physiological function specific to sleepiness. That’s the problem with reaction time – not specific enough. If this ideal behavioral biomarker could be established, it would predict future performance degradation with increasing sleep loss and would give decision makers (e.g. military commanders) actionable information. Right now variations of reaction time tests are the best thing we have, and perhaps application of statistical analysis to a number of different reaction time tests might produce something useful.
What behavioral tests can tell us
Researchers have tried to determine whether this degradation vulnerability correlates to other factors. Do individuals with quick reactions in psychomotor vigilance tests do better or worse than others when sleep deprived? Does the morningness-eveningness chronotype of the individual matter? Does age or sex matter? From research reports there is no apparent correlation to any of these characteristics.
PET scans of the brain can show relative amounts of blood flow – or at least glucose metabolism rate. When that rate is diminished in the prefrontal cortex, such as happens when a person first wakes up and has sleep inertia, measures of vigilance are diminished.
Sleep researchers Murray Johns of Epworth Sleep Centre, Melbourne, Victoria, Australia says he has identified 25 biomarkers, related to eyes and eyelids. By close examination of these physiological biomarkers, Johns attempts to predict impairment. While these tests do not indicate how long the subject has been awake (some people hold up to sleep deprivation better than others), they may pave the way to better tests in the future.