Caffeine and sleep
Sleep in humans can be affected by caffeine. There is an association between a daily intake of caffeine, reduced sleep quality, and increased daytime sleepiness35,36.
The most-documented effects of caffeine on sleep consist principally of prolonged sleep latency, shorter total sleep time, worsening of perceived sleep quality, increases in light sleep and shortening of deep sleep time, as well as more frequent awakenings. Rapid Eye Movement (REM) sleep is less affected35,36. REM sleep is a stage in the normal sleep cycle during which dreams occur and the body undergoes marked changes, including rapid eye movement, loss of reflexes, and increased pulse rate and brain activity.
The effects of caffeine on sleep depend not only on the amount of caffeine ingested at bedtime, but also on the amount of caffeine ingested over the whole day.
Human sensitivity to the effects of caffeine on sleep is variable and its exact basis is still debated. A 2016 systematic review of research on coffee, caffeine and sleep concluded that individuals will respond differently to caffeine based on a variety of factors, including age, sensitivity levels, regular coffee and caffeine intake, time of consumption and genetic variability36.
Normal sleep is divided into non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep is further divided into three progressively deeper stages of sleep: sleep begins in NREM and progresses through deeper NREM stages, before the first episode of REM sleep occurs approximately 80 to 100 minutes later. Thereafter, NREM sleep and REM sleep cycle over a period of approximately 90 minutes72.
The effect of caffeine on sleep
It is clear that caffeine intake can affect sleep, but a large intra-individual variability in the effects of caffeine is observed. A number of factors can affect the response to caffeine consumption and the subsequent impact on sleep, as outlined below.
Although the research in this area is limited, research to date indicates that there is a genetic variability in the metabolism of caffeine, and several genes have been identified that affect an individual’s sensitivity to caffeine30,73-75.
- The plasma concentration of caffeine after the ingestion of a given amount of caffeine may largely vary between subjects, suggesting a greater sensitivity of slow metabolizers73.
- Several genes have been identified that affect an individual’s sensitivity to caffeine. In humans, the distribution of distinct genotypes of the adenosine A2A receptor gene (ADORA2A) differs between self-rated caffeine-sensitive individuals with reduced sleep quality, and caffeine-insensitive individuals74. Both the DARPP-32 and PRIMA1 genes have also been implicated in caffeine sensitivity and caffeine induced insomnia respectively37. The same amount of caffeine can therefore affect two otherwise similar individuals differently, depending on their genetic make-up.
- Similarly, the probability of having a further genetic variation of the ADORA2A genotype decreases as habitual caffeine intake increases, suggesting that persons with this specific genotype may be less vulnerable to caffeine dependence75.
Only a few studies have evaluated the age-related effects of caffeine on sleep, and confounding factors are often present. Some research suggests that older adults may be more sensitive to the effects of caffeine. However, caffeine exposure may vary as a function of body weight. For example, older adults tend to consume the same amount of caffeine as younger adults but typically weigh less. Older adults may also self-limit the amount of caffeine they consume due to perceived sleep problems36.
- One study found that overall, caffeine produced similar effects in young adults (20-30 year-olds) and middle-aged subjects (45-60 year-olds). Only a few EEG spectral frequency bins were more affected by caffeine in middle-aged subjects than in young subjects76.
- A further study, by the same group, investigated daytime recovery sleep in the morning after 25 hours of wakefulness. Caffeine decreased sleep efficiency, sleep duration, slow-wave sleep and REM sleep during daytime recovery sleep similarly in both age groups. Middle-aged subjects showed greater decrements in sleep duration and sleep efficiency than young subjects during daytime recovery under placebo, compared to nocturnal sleep. Due to lower brain synchronization related to age and caffeine, these subjects had greater difficulty in overriding the circadian waking signal during daytime sleep and, as a result, had fragmented sleep38.
- A study of 22 young adults and 24 middle-aged adults concluded that caffeine increased sleep latency, shortened total sleep duration, and reduced sleep efficiency. The effects were more pronounced at a higher dose in middle-aged adults than in young adults. The higher dose of caffeine also increased absolute stage 1 sleep in young adults, whereas it decreased absolute stage 2 sleep in middle-aged adults. These results indicate that, compared to young adults, middle-aged adults are generally more sensitive to the effects of a high dose of caffeine on sleep quantity and quality39.
Habitual intake of caffeine
Research suggests that the effects of caffeine are less marked in those who regularly drink coffee when compared to occasional coffee drinkers40.
- Results from a research survey published in 2015 concluded that sleep quality was poorer in those who perceived themselves to be dependent upon caffeine, particularly amongst females. Caffeine dependence was associated with poorer sleep quality, increased daytime dysfunction, and increased levels of night-time disturbance77.
Time of consumption
Caffeine consumed closer to sleep time has the greatest potential for sleep disruption, although there are only limited studies assessing the timing of caffeine administration.
- A 2013 study assessed the impact of caffeine consumption on sleep at different periods before sleep, suggesting that caffeine consumed up to 6 hours beforehand may reduce total nightly sleep and reduce sleep quality41.
The effects of caffeine abstinence
A 2008 systematic review, including randomized trials, suggests that caffeine abstinence for a whole day could improve sleep quality and could be recommended by health practitioners when giving sleep hygiene advice42. The authors concluded that caffeine abstinence significantly lengthened sleep duration and improved sleep quality. Furthermore, subjects had less difficulty falling asleep on days when they drank decaffeinated coffee. However, people’s sensitivity to caffeine varies and individuals may or may not find that caffeine affects their sleep37.
The effect of caffeine on sleep in young people
Research suggests that caffeine helps restore better levels of wakefulness and counteracts degraded cognitive task performance due to sleep deprivation. However, caffeine may produce detrimental effects on subsequent sleep, resulting in daytime sleepiness, which may be a matter of concern, especially in adolescents78. Many adolescents use multiple forms of technology late into the night and concurrently consume caffeinated beverages to stay alert.
- A study looking at the effects of caffeine and technology on sleep duration and daytime functioning in young people showed that sleep was significantly related to the multi-tasking index79. Teenagers who scored 1.5-2 fold higher on multi-tasking indices slept less than 8-10 hours on school nights. Among the 33% of teenagers who fell asleep during school, caffeine consumption tended to be 76% higher than in those not falling asleep. The study suggests that, as a consequence, these teenagers were not fully functional throughout the day due to excessive daytime sleepiness, rather than because of the daytime effects of caffeine.
- Likewise, in a survey looking at adolescent caffeine use, it appeared that 95% used caffeinated drinks, primarily soft drinks or soda, but also coffee. In contrast with high soda users, mixed users who drank more coffee expected more energy enhancement from caffeine, and they were more likely to get up early and report more daytime sleepiness, which led to the use of caffeine to ‘get through the day’80.
- A study of a group of students during an exam period suggested that sleep quality and alcohol consumption significantly decreased, while perceived stress and caffeine consumption significantly increased. However, despite the fact that students shortened their time in bed and showed symptoms of insomnia, the authors concluded that sex, age, health status, as well as the amounts of alcohol and caffeine consumed had no significant influence on overall sleep quality81.
Caffeine, jet lag, and shift work
Jet lag and shift work sleep disorder can result in sleepiness and increase the risk of mistakes or injury. Shift work sleep disorder is a circadian rhythm sleep disorder characterized by insomnia and excessive sleepiness, affecting people whose work hours are scheduled during the typical sleep period.
According to the results of a systematic review, caffeine may be effective at improving performance in people who work shifts, or in those who are suffering from jet lag82. However, there is no data for subjects over the age of 40, who represent a population more likely to be affected. This research gap limits the conclusions that can be drawn and calls for more studies in this field.
Research in shift workers
- One intervention study has assessed counter measures for sleep-wake problems associated with night work, and suggested that a combination of napping and caffeine intake was best for improving alertness. A decrease in subjective sleepiness was also observed in individuals working a night shift following caffeine consumption83.
- Further work on the effect of caffeine consumption during night shifts suggests that caffeine increases alertness and clear-headedness after a period of wakefulness, but can also disturb subsequent daytime recovery sleep. Caffeine intake is associated with a higher core body temperature and this in turn is associated with a longer latency to sleep. Furthermore, sleep disturbances associated with caffeine consumed near the circadian trough of alertness are likely to still be present when daytime recovery sleep occurs around 5 hours later, potentially causing disturbed sleep84.
Caffeine and jet lag
Jet lag is often experienced after a long haul flight across different time zones and can cause extreme sleepiness or wakefulness at inappropriate hours. To counter jet lag, it helps to adjust to the new time zone quickly, sleeping, waking and eating at times appropriate to that area. Coffee consumption is associated with increased alertness and may help to manage feelings of sleepiness in those who experience jet lag.
A review of the research suggests that caffeine may be effective at improving performance in those who are suffering from jet lag85. However, for those taking short stopovers of 1-2 days in a different time zone, adapting to the local clock may not be the best strategy. One study suggests that sensible naps, combined with a moderate intake of caffeine during times of appropriate wakefulness and short-term use of sleeping aids, appear to be the most effective ways to maintain alertness and sleep in these situations85.
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