Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed
Sleep hygiene advice is widely given although few systematic studies have tested its empirical basis in the home environment. This study compared a fixed dose of caffeine (400 mg) taken at 0, 3 and 6 hours before habitual bedtime with placebo, using self-reported sleep at home and a validated portable sleep monitor.
Caffeine at each of the three times significantly disturbed sleep relative to placebo. The authors conclude that caffeine taken 6 hours before bedtime has important disruptive effects and that this supports refraining from substantial caffeine use for a minimum of 6 hours before bedtime.
Participants
Not described in the abstract (no sample size, age or sex).
Protocol
Fixed dose of caffeine (400 mg) at 0, 3 and 6 hours prior to habitual bedtime, compared with placebo.
Self-reported sleep in the home, plus objective monitoring with a validated portable sleep monitor.
Key Results
Caffeine at bedtime, 3 hours before bedtime or 6 hours before bedtime each had significant effects on sleep disturbance relative to placebo (p < 0.05 for all).
The authors point to the magnitude of the reduction in total sleep time with caffeine taken 6 hours before bedtime; the size of that reduction is not given in the abstract.
Practical Application
A fixed 400 mg dose of caffeine taken at bedtime, 3 hours before or 6 hours before habitual bedtime each significantly disturbed sleep compared with placebo (p < 0.05 for all), measured at home with a diary and a portable sleep monitor; the abstract gives no sample size. To "how late can I have caffeine?": this study supports stopping substantial caffeine a minimum of 6 hours before bed — and 6 hours was still not enough to leave sleep untouched.
The 6-hour figure is a floor, not a safe line: the earliest time tested still disturbed sleep, and no longer interval was tested.
Inference, not tested here: a caffeinated evening session or late race carries a sleep cost that night; smaller doses were not studied, so the cost of a single gel or a small coffee is unknown from this paper.
Limitations
The abstract gives no sample size, no participant description and no numerical size of the sleep loss.
One dose (400 mg), not scaled to body mass; habitual caffeine use is not described.
Not athletes; no exercise and no next-day performance.
No interval longer than 6 hours was tested.
Written from the abstract and record metadata only (full text not read).
How this entry was checked and changed
Why this level
placebo-controlled; sample size and randomisation are not given in the abstract