Drinking water before coffee on waking makes sense, since you genuinely lose water overnight — but the idea that you then need to wait 90 minutes before your first caffeine to let the wake-up cortisol spike settle and dodge an afternoon crash has never been directly tested.

noli's read

Overnight water loss through breathing and perspiration is real, and drinking a glass of water first thing is a reasonable, low-risk habit. The shakier part concerns cortisol: the wake-up cortisol spike is well documented, but no controlled trial has directly tested whether delaying caffeine by 90-120 minutes improves energy, mood, or sleep — that's a mechanistic inference, not a validated protocol. A recent sports-nutrition review points out that the argument actually runs backwards on adenosine's real kinetics: it rises within minutes of waking rather than continuing to decline, which undercuts the central logic of the 90-minute rule. Another important nuance: in regular coffee drinkers, the cortisol response to caffeine is markedly blunted, and can disappear entirely at high habitual intake — making the 'cortisol' worry largely moot for most daily drinkers. The rule's popularizer, neuroscientist Andrew Huberman, has himself walked back the recommendation from its original strict form. The caffeine lever actually backed by solid research isn't the morning clock — it's the evening cutoff: avoiding caffeine in the 8-9 hours before bed has a clear, measured effect on sleep, well beyond anything morning timing can deliver.

the sources

1. 1Lovallo WR, Whitsett TL, al'Absi M, et al. 2005
2. 2Lovallo WR et al. 2006
3. 3Gardiner C et al. 2023