Sleepiness, troika of consciousness cycle, and the Epworth sleepiness scale

Excessive daytime sleepiness (EDS) is an important indicator when diagnosing sleep-disordered breathing and evaluating its treatment results. However, there appears to be some confusion as to what exactly is sleepiness; Dorland's Illustrated Medical Dictionary does not help. The medical literature was reviewed in order to assemble a schematic model that would suggest a definition of sleepiness and how it can be measured. The derived model is entitled the troika of consciousness cycle (TCC). It assumes that the presence of wakefulness, nonrapid eye movement sleep (NREMS), and rapid eye movement sleep (REMS) is determined by the interactions of four drives: two promoting wakefulness and one each for the two sleep states. The TCC illustrates that inadequate sleep results in sleep debt, but that sleepiness is determined solely by the nearness of the secondary wake drive line to the NREMS drive line. Contact of these lines indicates dozing, a change in consciousness state, an observable event. The probability of this event may be defined as objective sleepiness; this is what the Epworth sleepiness scale (ESS) attempts to measure. Studies indicate that the ESS can determine EDS with greater sensitivity and selectivity than either the multiple sleep latency test or the maintenance of wakefulness test.