A Reader's Guide to the PADA Hypothesis Photoperiod–Adenosine–Receptor Desensitization–Aging: An Introduction and Reading Guide for New Readers

Conventional wisdom holds that sleepiness is determined by the accumulated concentration of adenosine in the brain—the more adenosine, the sleepier one feels. This paradigm explains most everyday observations, yet it struggles with three classes of reported phenomena: some individuals remain awake for multiple consecutive days without proportionally rising subjective sleepiness; natural short sleepers sleep only 4–6 hours per day while staying healthy; and population-level sleep duration shows a U-shaped—not linear—relationship with mortality risk. The PADA hypothesis (Photoperiod–Adenosine–Desensitization–Aging) offers a candidate explanation: what governs sleepiness and acute stress signaling is not the absolute concentration of adenosine, but the degree to which adenosine receptors have been 'desensitized'—once receptors are phosphorylated by GRK kinases, even abundant adenosine can no longer drive the G-protein downstream drowsiness and stress signals at full strength. Written for non-specialist readers, this guide introduces in turn the adenosine–sleep circuit, the two forms of receptor desensitization (membrane-retained D1 and internalization/degradation D2), candidate photoperiodic pathways that shape the adenosine waveform via the circadian system, and the link between long-term desensitization and aging signals; it also states explicitly the three prerequisites for the hypothesis and the core falsifiability criteria. The document can be read on its own; Section 7 describes the relationship between the two archived full texts and suggested reading paths.