Caffeine before bedtime has little effect on sleep quality, but alcohol and nicotine does

By Sally Robertson, B.Sc.Reviewed by Kate Anderton, B.Sc. (Editor)Aug 8 2019

A new study has found that drinking coffee close to bedtime does not affect the quality of sleep, but that evening alcohol intake and smoking are the main culprits that disrupt sleep. The research is one of the largest longitudinal studies conducted so far to assess how evening use of the substances affects sleep quality.

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The research, which was conducted at Florida Atlantic University and Harvard Medical School, showed that drinking alcohol within four hours of going to bed was significantly more likely to interfere with sleep than drinking caffeine.

Nicotine intake during the evening, whether it be through vaping or smoking a regular cigarette, was even more strongly associated with disrupted sleep. When it came to consuming coffee or tea, the researchers were surprised to find no association with poor quality sleep.

The findings are particularly enlightening for African-Americans

The study, which followed 785 African-Americans for 14 years, is particularly enlightening for African Americans, who tend to suffer from insomnia, sleep apnea and other sleep-related disorders more than other ethnic or racial groups.

African Americans have been underrepresented in studies examining the associations of nicotine, alcohol, and caffeine use on sleep. This is especially significant because African Americans are more likely to experience short sleep duration and fragmented sleep compared to non-Hispanic Whites, as well as more deleterious health consequences associated with inadequate sleep than other racial or ethnic groups."

Christine Spadola, Lead Author

Despite this, previous studies have been limited

Between 50 to 70 million Americans suffer from sleep disorders, which are associated with various negative health effects including heart disease, diabetes, hypertension and some forms of cancer.

The previous understanding has been that evening intake of alcohol, caffeine and nicotine negatively impacts sleep quality.

However, many studies examining these associations have been limited by small study samples that poorly represent objective measures of sleep and different racial and ethnic groups. Furthermore, such research has often been carried out in laboratory or observatory settings.

Given the negative impact that poor sleep can have on health and how widespread the use of these substances is, relatively few studies have properly investigated the association between sleep parameters and the evening use of alcohol, caffeine and nicotine.

One of the largest longitudinal examinations to date

For the current study, the researchers monitored 785 African Americans over a total of 5,164 days and nights using actigraphy (wrist-watch-like sensors) and daily sleep diaries. They examined the night-to-night associations between evening intake of caffeine, alcohol and nicotine within four hours of bedtime and the following sleep quality parameters: sleep duration, sleep efficiency and waking after sleep onset. None of the participants suffered from clinical sleep disorders.

The study, which also involved researchers from Brigham and Women's Hospital, Harvard T. H Chan School of Public Health, Emory University, the National Institutes of Health, and the University of Mississippi Medical Center, represents one of the largest longitudinal investigations to date to examine evening consumption of the substances and objectively measured sleep outcomes among African-Americans while they are in their natural environments.

Nicotine and alcohol significantly affects sleep quality

As reported in the journal Sleep, nicotine and/or alcohol intake within four hours of bedtime was associated with worse sleep continuity than when the substances were not used during this timeframe.

The association remained even after controlling for other factors that could affect sleep such as age, gender, obesity, education level, having work/school the next day and symptoms of depression, anxiety or stress.

Nicotine intake was the factor most strongly associated with sleep disruption and in relation to sleep duration, there was a significant interaction between evening nicotine use and insomnia. Among participants with insomnia, evening nicotine intake was associated with an average 43-minute reduction in sleep duration.

No such effect was observed for caffeine intake

Caffeine intake, on the other hand, seemed to have no such effect: “We did not observe an association between ingestion of caffeine within four hours of bed with any of the sleep parameters,” says Spadola.

Independent sleep expert and author of “How to Sleep Well,” Neil Stanley, supports the study’s findings, saying the idea that drinking coffee before bed will keep you awake at night is a myth.

“Some people are very sensitive to the effects of caffeine and for these people it’s important to avoid drinking beverages containing caffeine too close to bedtime - but there is no golden rule about this, just listen to your body,” he says.

“For some people the effects caused by caffeine are much lower and may not have any effects at all.”

With so many contradictory studies on sleeping patterns, can we trust the data?

The authors of the current study note that self-reported data is subject to error, but say they are still confident in the consistency of their findings.

The results were observed among individuals unselected for sleep problems and who generally had high sleep efficiency. Furthermore, they were based on longitudinal data, meaning the associations identified not only accounted for between-person differences but also within-person variations in exposures and covariates such as age, gender, obesity, education level, having work/school the next day and mental health symptoms.

The researchers conclude:

Nicotine and alcohol use within 4 hours of bedtime were associated with increased sleep fragmentation in the associated night, even after controlling for multiple potential confounders.  These findings support the importance of sleep health recommendations that promote the restriction of evening alcohol and nicotine use to improve sleep continuity.”