Study Summary
Many people enjoy caffeine throughout the day, but its effects on sleep can vary depending on how much is consumed and when. This study closely examined how different doses and timings of caffeine intake influence both subjective and objective sleep quality.
What They Did
In a randomised, placebo-controlled crossover trial by Gardiner et al. (2025), 23 healthy men aged 18 to 40 with moderate caffeine use (less than 300 mg/day) were given either 100 mg or 400 mg of caffeine, or a placebo, at 12, 8, or 4 hours before bedtime. Their sleep was assessed using at-home polysomnography and sleep diaries across seven separate conditions.
To give context:
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100 mg caffeine ≈ 1 regular cup of coffee, 1–2 espresso shots, or 2.5 cans of cola.
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400 mg caffeine ≈ 4 cups of brewed coffee, 5–6 espresso shots, 10 cans of cola, or one strong energy drink or pre-workout supplement.
What They Found
The 100 mg dose did not affect sleep at any time point, including when consumed just 4 hours before bedtime.
In contrast, the 400 mg dose significantly disrupted sleep:
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At 4 hours before bed: longer time to fall asleep, more awakenings, reduced deep (N3) sleep, and worse perceived sleep quality.
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At 8 hours: more time awake after falling asleep, and lower sleep efficiency.
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At 12 hours: reduction in deep sleep still occurred, even though participants did not feel much difference.
Interestingly, most people were unable to tell how much their sleep had been impacted, especially when caffeine was consumed earlier in the day.
Why It Matters
The study confirms that both caffeine dose and timing are critical for sleep. A small dose (100 mg) is unlikely to cause problems, even when taken in the late afternoon. However, 400 mg — the equivalent of strong pre-workouts or several coffees — can still disturb sleep even if taken 12 hours earlier.
This challenges the vague advice to “avoid caffeine in the evening” and supports more specific guidelines based on dosage.
My Clinical Insight
Many patients are surprised when I suggest that their morning or early afternoon caffeine might be affecting their sleep. This study confirms that low doses (100 mg) are generally safe, even 4 hours before bed, but that higher doses (400 mg) can cause measurable sleep disruption — even without you noticing it.
Still, it’s worth noting that this study had limitations. It included only 23 young, healthy men with moderate caffeine habits. We can’t assume the same results apply to women, older adults, or people with sleep disorders or different metabolisms. But it gives us strong, practical guidance: if you’re struggling with sleep, reducing both the amount and timing of caffeine is a smart place to start.
Until sleep feels natural again, Dr Noaman
Reference:
Gardiner et al., Dose and timing effects of caffeine on subsequent sleep, Sleep, 2025
