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Sleepiness is managed by two separate mechanisms

Your brain tracks hours spent awake and time of day using two different systems. Fatigue happens when these two signals contradict each other.

The principle

Sleep pressure builds constantly while you stay awake

From the moment you wake up, a chemical debt accumulates continuously in the brain. The longer you stay awake, the heavier this baseline pressure feels.

Sleep pressure

Your body clock dictates alertness on a schedule

An internal master clock drives waves of alertness and drowsiness throughout the day. This cycle repeats on a 24-hour loop regardless of how much sleep you had.

Circadian rhythm

System conflict explains crashes and second winds

An afternoon crash occurs when high sleep pressure meets a temporary dip in clock-driven alertness. Late-night surges happen when the internal clock temporarily overrides heavy sleep pressure.

The clash

Idea

Two Distinct Systems Control Your Daily Sleepiness

Fatigue occurs when your internal clock and sleep pressure pull in opposite directions.

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@spot #sleep #biology #circadian #neuroscience

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