Firstly, it is helpful to understand why caffeine affects sleep. Caffeine can be consumed as a food (e.g. chocolate, workout bars) or a beverage (e.g., tea, coffee, energy drinks, flavoured milk drinks). Caffeine is a powerful stimulant that enhances mood, alertness, and thinking. It has a half-life of 4-6 hours. This means our body will have processed 50% of the caffeine consumed within 4-6 hours, and the stimulating effects of the other 50% will still be going strong. Caffeine blocks the effects of adenosine on our brain, which is produced while we are awake. Adenosine is necessary to initiate and maintain sleep. The longer we are awake, the more adenosine is in our system, and the stronger our drive is to sleep. By blocking adenosine, caffeine masks the natural signals to our body that it is time to sleep.
Summary Slide – Dose & Timing of Caffeine Intake
Summary
Objective:
This study investigated how consuming either a moderate (100 mg) or high (400 mg) dose of caffeine at different times of day (12, 8, or 4 hours before bedtime) affects sleep quality.
Methods:
- Twenty-three men (average age 25.3) with moderate caffeine use (< 300mg/day) participated.
- Each person underwent 7 different conditions in a randomised, double-blind, placebo-controlled crossover design.
- Sleep was assessed using in-home sleep recordings and diaries.
- The study used linear mixed models to analyse effects.
Results:
- 100 mg caffeine had no significant impact on sleep at any time point.
- 400 mg caffeine:
- Disrupted sleep when taken 12, 8, or 4 hours before bedtime.
- Caused delayed sleep onset, changes in sleep architecture, and greater fragmentation (especially at 8 hours).
- Perceived sleep quality dropped significantly only when taken 4 hours before bed.
Conclusions:
- 100 mg caffeine appears safe up to 4 hours before bedtime.
- 400 mg can negatively affect sleep up to 12 hours after intake, and the effects worsen the closer it is consumed to bedtime.
- There is a mismatch between perceived and actual sleep quality, suggesting that people may underestimate how caffeine affects their sleep.