RCTHumann = 212024
Concentration-effect relationships of plasma caffeine on EEG delta power and cardiac autonomic activity during human sleep.
Plain-language summary
This study set out to measure how the amount of caffeine in the blood tracks with sleep depth and heart activity. Twenty-one healthy young men took either 160 mg of caffeine or a dummy pill in a slow-release form at bedtime, and were recorded through a 4 hour sleep window starting four and a half hours later. Higher blood caffeine went with less delta wave activity, the brain pattern of deep sleep, and with a lower heart rate and more parasympathetic activity (the rest-and-digest side of the nervous system, which slows the heart and normally rises during calm sleep). The more caffeine in the blood, the larger each effect. Another study here, on daily caffeine habits and sleep, comes from the same research group.
Key findings
- Caffeine reduced EEG delta activity (deep sleep brain waves) during NREM sleep.
- Caffeine lowered heart rate by an average of 3.24 beats per minute.
- Caffeine increased high-frequency heart rate variability, a marker of parasympathetic activity (the rest-and-digest side of the nervous system, which slows the heart).
- Effects were dose-dependent, with specific caffeine concentration thresholds for each effect.
How to analyze the study yourself
- Study type
- Randomized controlled trials assign people to groups by chance, so differences between groups are unlikely to come from who chose what, but the small sample size and short follow-up limit how broadly the findings can be applied.
- Population
- Healthy young men, so results may not generalize to women, older adults, or those with health conditions.
- Funding
- Not reported
- Takeaway
- This study found that caffeine in the blood reduced deep-sleep brain waves and lowered heart rate, while raising parasympathetic activity, the rest-and-digest side of the nervous system that slows the heart. The higher the blood level, the larger each effect. If caffeine disturbs your sleep, the amount still in your blood at bedtime is what matters, so move it earlier in the day.