A recent study published in Sleep sheds light on the relationship between caffeine consumption and its impact on sleep. Researchers found that while a 100 mg dose of caffeine (roughly equivalent to one cup of coffee) can be consumed up to four hours before bedtime without significant effects on sleep, a 400 mg dose (comparable to four cups of coffee) disrupts sleep when taken up to 12 hours before bedtime. Importantly, the findings reveal that people often struggle to recognize the sleep disruptions caused by caffeine, especially when it is consumed earlier in the day.
Caffeine is a widely consumed stimulant used to enhance alertness and counteract sleep deprivation. While prior research has highlighted caffeine’s general impact on sleep, it lacked clarity on how dosage and timing interact. The new study aimed to fill that gap by exploring how different doses of caffeine affect sleep when consumed at various intervals before bedtime.
With an estimated 40% of people worldwide struggling to get sufficient sleep and increasingly relying on caffeine to manage fatigue, understanding the specific timing and quantity that disrupts sleep is essential.
“We were interested in this topic because there is limited evidence to guide clear recommendations on the dose and timing of caffeine consumption relative to sleep,” said study author Carissa Gardiner, a postdoctoral researcher at the SPRINT Research Centre at the Australian Catholic University. “With approximately 80% of the population consuming caffeine, it may contribute to the high rates of sleep insufficiency currently reported. Understanding how caffeine dose and timing of intake affect sleep can help develop practical guidelines to minimise the negative impact on sleep.”
The researchers conducted a placebo-controlled, double-blind, randomized study involving 23 healthy male participants aged 18–40 years. To participate, individuals had to be moderate caffeine consumers (less than 300 mg per day) with no medical conditions or habits that might affect sleep. Female participants were excluded to avoid confounding effects related to hormonal cycles.
Each participant completed seven experimental conditions over a 21-day period. These conditions included consuming either 100 mg or 400 mg of caffeine—or a placebo—at 12, 8, or 4 hours before their typical bedtime. Participants followed a consistent sleep schedule, confirmed through wearable monitors and sleep diaries. During the experiment, caffeine was administered in capsules, and saliva samples were collected to verify caffeine levels in the body. Sleep was assessed both objectively, using a validated sleep-monitoring device, and subjectively, through participant-reported sleep quality and duration.
The findings highlighted a dose-dependent relationship, with higher doses causing more pronounced disruptions to sleep. Specifically, a 100 mg dose had no measurable impact on either objective or subjective sleep outcomes, even when consumed as little as four hours before bedtime. In contrast, a 400 mg dose caused disruptions to sleep even when consumed up to 12 hours before bedtime.
“Consuming 400 mg of caffeine within 12 hours of bedtime can negatively impact sleep, with the effects worsening the closer caffeine is consumed to bedtime,” Gardiner told PsyPost.
When participants consumed 400 mg of caffeine 12 hours before bedtime, they experienced noticeable disruptions to their sleep architecture, particularly a reduction in deep, restorative sleep (non-rapid eye movement stage 3). At this time point, total sleep time was modestly reduced, and participants experienced slightly more awakenings during the night.
These effects became more pronounced when the caffeine was consumed eight hours before bedtime. Sleep efficiency—calculated as the percentage of time spent asleep while in bed—dropped by 7%, and participants spent more time awake after initially falling asleep. This timing also led to an increased number of nighttime awakenings and further reductions in the duration of deep sleep.
The most significant sleep disruptions occurred when participants consumed 400 mg of caffeine four hours before bedtime. Sleep onset latency—the time it takes to fall asleep—increased by an average of 14 minutes, total sleep time was reduced by approximately 50 minutes, and deep sleep duration decreased substantially.
Sleep fragmentation also increased, with more frequent awakenings and a higher proportion of lighter sleep stages. Subjective assessments aligned with these findings; participants reported feeling less rested and experiencing lower sleep quality when they consumed 400 mg of caffeine four hours before going to bed.
Interestingly, participants had difficulty perceiving the full extent of sleep disruptions caused by caffeine, particularly when it was consumed 8 or 12 hours before bedtime. Although objective measures revealed significant increases in nighttime awakenings and reductions in deep sleep, participants’ subjective reports of sleep quality and restfulness did not consistently reflect these disturbances. This mismatch between objective and perceived sleep quality underscores the challenge consumers face in recognizing caffeine-induced sleep fragmentation, especially when consumption occurs earlier in the day.
“The mismatch between objective and subjective sleep was particularly interesting,” Gardiner said. “Many people believe caffeine doesn’t affect their sleep, but our findings suggest it may disrupt sleep even when individuals don’t perceive it.”
The researchers also examined genetic variations related to caffeine metabolism and sensitivity but was not large enough to draw definitive conclusions. Larger studies could better explore how genetics mediate individual responses to caffeine.
“My long-term goal is to continue building the evidence base to provide consumers with clear, evidence-based guidance on how to minimize caffeine’s impact on their sleep,” Gardiner said.
The study, “Dose and timing effects of caffeine on subsequent sleep: a randomized clinical crossover trial,” was authored by Carissa L Gardiner, Jonathon Weakley, Louise M. Burke, Francesca Fernandez, Rich D. Johnston, Josh Leota, Suzanna Russell, Gabriella Munteanu, Andrew Townshend, and Shona L. Halson.
