What chemical accumulates to signal sleep need?

• A. Dopamine
• B. Glutamate
• C. Adenosine
• D. Norepinephrine

Answer: (C)

Adenosine accumulation is the brain’s signal of sleep need. As you stay awake, ATP is broken down and adenosine builds up in the brain, especially in areas that regulate wakefulness. This rising adenosine dampens activity in arousal networks and promotes the feeling of sleepiness, creating sleep pressure. During sleep, adenosine is cleared, reducing that pressure and helping you wake up refreshed.

The other substances listed are more about promoting or maintaining wakefulness. Dopamine, glutamate, and norepinephrine support alertness and neural activation rather than signaling that sleep is overdue. Caffeine works by blocking adenosine receptors, which is why it can lessen sleepiness.