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StillnessSleep fact

Caffeine keeps you awake by blocking a sleep signal, not by removing it. A substance called adenosine builds up in the brain while you are awake and makes you drowsy; caffeine occupies its receptors for a few hours, but the adenosine is still there.

Sleep science

HealthScience

The story behind the card

Adenosine is a by-product of the energy cells use, and it gradually accumulates in the brain during waking hours, damping the activity of nerve cells that keep you alert. In the two-process model of sleep, proposed by Alexander Borbély in 1982, this growing sleep pressure is called 'process S', and it works together with the body's circadian clock, its roughly 24-hour internal rhythm. Caffeine's shape is close enough to adenosine's that it can occupy the same receptors without activating them, so it dulls sleepiness without reducing the need for sleep. Its half-life in adults, the time the body takes to clear half of it, is often around five hours but varies widely. When it wears off, the adenosine that kept building can act again, one reason for the slump some people call a caffeine crash.

Türkçe oku
Kafein uyku sinyalini ortadan kaldırmaz, yalnızca önünü keser. Uyanık kaldıkça beyinde adenozin adlı bir madde birikir ve uyku getirir; kafein birkaç saatliğine onun bağlandığı yerleri tutar, ama adenozin orada olmaya devam eder.

Adenozin, hücrelerin enerji kullanırken ortaya çıkardığı bir yan üründür. Uyanık geçen saatler boyunca beyinde yavaş yavaş birikir ve uyanıklığı sağlayan sinir hücrelerinin etkinliğini azaltır. Alexander Borbély'nin 1982'de önerdiği iki süreçli uyku modelinde, giderek artan bu uyku baskısına “S süreci” denir; bu süreç, bedenin aşağı yukarı 24 saatlik iç ritmi olan sirkadiyen saatle birlikte işler. Kafein molekülü adenozine o kadar benzer ki aynı reseptörlere yerleşebilir ama onları çalıştırmaz; bu yüzden uykululuğu bastırır, uyku ihtiyacını azaltmaz. Yetişkinlerde kafeinin yarı ömrü, yani vücudun yarısını temizlemesi için geçen süre, genellikle beş saat civarındadır ama kişiden kişiye çok değişir. Etkisi geçince bu arada birikmeye devam eden adenozin yeniden iş başına geçer; bazı insanların “kafein çöküşü” dediği ani düşüşün bir nedeni budur.

Sources and further reading

Background reading: Adenosine
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