You’ve seen the rule: wait 90 minutes after waking before your first coffee, or you’ll crash in the afternoon.

Andrew Huberman popularized it in 2022 and it’s been everywhere since. Nobody has actually tested it.

The reasoning is that adenosine, the molecule that makes you sleepy, is low when you wake up, so coffee right away supposedly sets you up for a crash later. A 2024 review in the Journal of the International Society of Sports Nutrition went through the claim and found no evidence that caffeine on waking causes an afternoon crash. It called the adenosine part “scientifically inaccurate and not supported by research.”

The cortisol version doesn’t hold up either. Cortisol follows its own daily rhythm and rises in the first hour after you wake. Caffeine raises cortisol whether you drink it at 6:30 or 8:00. Per the same review, the increase “will still happen if you delay intake.” Waiting doesn’t protect anything.

To be fair to the rule: waiting isn’t harmful. Sleep researcher Michael Grandner (University of Arizona) says there’s no harm in coffee first thing, and he waits 30 to 60 minutes himself because he likes it that way. If waiting feels better, wait. Just don’t call it science. And if you slept badly, caffeine works best early, since it helps most with sleepiness after short sleep.

Now the end of the day, where there is data.

In a 2013 trial in the Journal of Clinical Sleep Medicine, 12 healthy sleepers took 400 mg of caffeine (roughly 2 to 3 cups) at 0, 3, or 6 hours before bed. Even at 6 hours, measured sleep time dropped by more than an hour. The participants didn’t notice.

It’s a small study and a big dose, so don’t treat the exact numbers as gospel. But it points at the lever that matters: when you stop, not when you start.

What to do with that:

• Drink your first coffee whenever you want it.
• Set a cutoff. The authors suggest nothing after about 5 pm. If you go to bed early or you’re caffeine-sensitive, make it earlier.
• If you crash at 3 pm, look at last night’s sleep before you blame this morning’s coffee.

The start of your caffeine window barely matters. The end of it does.

This is research I read, not medical advice. If you’re pregnant, anxious, or have a heart condition, ask your doctor about caffeine.

P.S. If you want this kind of detail applied to your own schedule, training and nutrition, I take on 1-on-1 coaching clients. That’s a plan built around your week, your goals and your macros, not some generic "AI" template.

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