Which of the following best lists the three main components of fatigue?

• A. Sleep quantity; Circadian rhythm; Post-sleep grogginess
• B. Sleep reservoir; Circadian rhythm; Post-sleep grogginess
• C. Circadian rhythm; Sleep debt; Post-sleep grogginess
• D. Sleep reservoir; Sleep debt; Post-work fatigue

Answer: (B)

Fatigue comes from a trio of interacting processes: the homeostatic sleep drive, the circadian timing system, and post-awakening grogginess. The sleep reservoir concept represents the stored sleep need built up during wakefulness and replenished by sleep; when this reservoir is depleted, fatigue and performance decline. The circadian rhythm modulates alertness across the day, causing natural highs and lows independent of how much sleep you’ve had, so fatigue can spike at certain times even with adequate rest. After waking, sleep inertia or grogginess is the temporary cognitive and sensory slowdown that can linger, especially if you wake from deep sleep or with insufficient sleep; this directly adds to fatigue immediately after waking. Together, these three components explain why you feel tired at different times, after poor sleep, and right after waking. The other options either mix in less precise terms for the homeostatic drive or add factors like post-work fatigue that aren’t central to the fatigue construct described here.