Did you know that Alexander Borbély not that long ago published a paper called 'The two-process model of sleep regulation: Beginnings and outlook'? The paper recaps the research and series of events that led to the modelling present in the fatigue models used within aviation. From the paper:
The two-process model serves as a major conceptual framework in sleep science. Although dating back more than four decades, it has not lost its relevance for research today. Retracing its origins, I describe how animal experiments aimed at exploring the oscillators driving the circadian sleep–wake rhythm led to the recognition of gradients of sleep states within the daily sleep period. Advances in signal analysis revealed that the level of slow-wave activity in non-rapid eye movement sleep electroencephalogram is high at the beginning of the 12-light period and then declines. After sleep deprivation, the level of... (Read the full article here.)