Caffeine

What It Is

How much caffeine you take, and how late you take it.

Why It Matters

In controlled trials, caffeine measurably degrades the night that follows it. Two meta-analyses, drawing on a largely overlapping set of primary studies, found that it lowered sleep efficiency, lengthened the time taken to fall asleep, and cut deep sleep. Those trials dosed a single night at a time, well above a single cup, so read them as what a heavy or late dose can cost you, not as the price of your usual coffee.

The mechanism is adenosine: caffeine blocks the receptors that register the sleep pressure building through your day, muting the signal to fall and stay asleep for as long as the drug is circulating. Dose and timing both matter, though where the line sits is unsettled. One trial in young men found a high dose (roughly four coffees) still shortened deep sleep taken twelve hours before bed, while a single-cup dose did nothing detectable four hours out. The evening dose is the one you can feel: in that trial only the dose taken four hours before bed dented perceived sleep quality, while the same amount taken in the morning shortened deep sleep without registering at all.

Habitual drinkers seem to stop noticing rather than stop losing. Large studies find little or no link between how much coffee people drink and how well they say they sleep, but the one that measured sleep at home with electrodes found deeper sleep at four or more caffeinated drinks a day, which its authors read as adaptation rather than damage. Keep caffeine to the earlier part of your day, keep anything late small, and if you sleep badly without knowing why, moving your last cup earlier for a couple of weeks is a cheap thing to test.

• Gardiner, C. et al. (2023). The effect of caffeine on subsequent sleep: A systematic review and meta-analysis. Sleep Medicine Reviews. doi.org/10.1016/j.smrv.2023.101764
• Gardiner, C.L. et al. (2025). Dose and timing effects of caffeine on subsequent sleep: a randomized clinical crossover trial. Sleep. doi.org/10.1093/sleep/zsae230
• Chang, Y.H. et al. (2025). Age- and dose-specific effects of caffeine on sleep: A meta-analysis of controlled crossover trials. Sleep Medicine. doi.org/10.1016/j.sleep.2025.106874
• Stucky, B. et al. (2025). Community-based causal evidence that high habitual caffeine consumption alters distinct polysomnography-derived sleep variables. Journal of Psychopharmacology. doi.org/10.1177/02698811251368364
• Söderholm, S. et al. (2026). Habitual coffee consumption poorly correlates with sleep quality and daytime sleepiness: A cross-sectional study. PLoS One. doi.org/10.1371/journal.pone.0344479