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Subjective sleepiness and fatigue ratings were dependent on the number of continuous hours awake and circadian phase. Self-reported sleepiness significantly increased with time awake on both the KSS (A; P < 0.0001) and the ANAM Fatigue scale (C; P = 0.0001). Both measures also exhibited a robust circadian rhythm, with highest sleepiness reported when subjects were awake during the biological night (B, P = 0.0001; and D, P = 0.0003).
