Caffeine amount and timing both matter for sleep.
A small crossover trial tested two amounts at three times. Its strongest sleep loss followed the larger, later amount.
The 30-second takeaway
In this small trial, 400 mg of caffeine four hours before bed reduced sleep by about 51 minutes compared with placebo. That amount matters: the result does not describe every coffee. The researchers found no significant sleep effect from 100 mg here, but that is not a universal safe amount or cutoff.
This was a small study of young men. Individual responses and other populations may differ; the trial does not establish a universal cutoff.
What researchers found
Total sleep time
400 mg four hours before bed versus placebo; objective overnight measurement. This was the larger tested amount.
The same people, different conditions
Each participant tried the caffeine and placebo conditions, separated by washout periods. Researchers recorded sleep with a home monitoring device.
Beyond a bedtime rule
Effects varied by amount, timing and sleep measure. Some objective changes were less obvious in participants’ own sleep ratings.
The original publication
Dose and timing effects of caffeine on subsequent sleep: a randomized clinical crossover trial
Carissa L. Gardiner et al.
Sleep · 2025
- PubMed ID
- 39377163
- Record checked
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