Why does caffeine make me tired?

Caffeine hides tiredness instead of clearing it. It blocks the signal your brain uses to feel sleepy, while the cause of that signal keeps building. When the caffeine fades, the tiredness it was covering is still there, and there’s more of it.

It feels backwards. You drink coffee to stay awake, and an hour or two after it wears off you’re more tired than before. People take that as proof the coffee “crashed” them, and reach for another one.

We’d answer the afternoon slump with a small cup after lunch, never a big one at 4pm. A big coffee late in the day borrows from tonight, and tonight is where tomorrow’s tiredness comes from.

It’s like switching off the low-battery warning on your phone. The warning goes away, and the battery keeps draining.

Reason one: the tiredness was waiting

From the moment you wake up, a chemical called adenosine builds up in your brain, and the more of it there is, the sleepier you feel. Caffeine fits into the receptors adenosine uses and blocks them, so the sleepy signal doesn’t land. It does nothing to the adenosine itself, which keeps building all day. When the caffeine wears off, the receptors open up to a brain that’s been piling up adenosine the whole time.

Reason two: last night’s sleep

Across 24 studies, caffeine cut total sleep by about 45 minutes and deep sleep by about 11 minutes, while falling asleep took only 9 minutes longer. You don’t notice the loss at night. You feel it the next day, have more coffee, and lose more sleep. The more of that coffee lands in the afternoon, the more it costs the night.

Reason three: withdrawal between cups

If you drink caffeine every day, your body adapts to it. A 2004 review of 66 studies found that when the usual dose is late or missing, withdrawal starts 12 to 24 hours after the last one, and its symptoms include fatigue, low energy, drowsiness and feeling foggy. Some of the tiredness before your first coffee, or on a day you have less, can be withdrawal. It showed up even in people stopping as little as 100 mg a day.

What about the afternoon crash?

The popular advice to wait 90 minutes after waking for your first coffee claims to prevent an afternoon crash. A 2024 review of the research found no evidence that a coffee right after waking causes a crash, or that waiting prevents one. The cup that changes how you feel tomorrow is the last one of the day (wait 90 minutes for coffee?).

What to do

Keep your last cup small and earlier, so it doesn’t cost the night. If you want to stop relying on it, step down slowly instead of quitting at once (how to quit caffeine without a headache). The cutoff clock shows what your current afternoon leaves at bedtime, and how to have more energy without caffeine has what clears the tiredness instead of covering it.