Clinical ReviewCaffeine: Sleep and daytime sleepiness
Introduction
Caffeine is one of the most commonly consumed psychoactive substances in the world. It is available in a variety of dietary sources such as coffee, tea, coca, candy bars, and soft drinks. It also is an ingredient in various over-the-counter drugs (OTCs) including headache, cold, allergy, pain relief, and alerting drugs. The caffeine content of some of the various beverages, foods, and OTCs is provided in Table 1. The table is not to be considered exhaustive. The caffeine content of foods, commercially prepared beverages, and OTCs is constant and documented, but the caffeine content of brewed beverages can vary depending on the bean used and the method of brewing. This variability requires that investigators estimate the caffeine content of brewed beverages when assessing self-reported caffeine consumption and introduces error variance when relating caffeine doses to any outcome variables.
Caffeine's effects on laboratory assessed sleep in double-blind placebo controlled studies have been well documented. Laboratory studies have also documented its alerting and performance-enhancing effects. However, the extent to which regular dietary caffeine intake affects sleep and daytime function in the population is not fully known. Such information is important since there is evidence suggesting the use of caffeine in society is expanding, both in terms of increased daily dosages and earlier ages for the initiation of regular daily caffeine use.
To understand the effects of caffeine and its discontinuation on sleep and daytime alertness and its tolerance and dependence liability we will first review its pharmacology. After reviewing the pharmacology of caffeine and the well-documented sleep disruptive effects of caffeine and studies suggesting that caffeine's performance-enhancing effects are for the most part restoring performance degraded by sleepiness, this review will evaluate the degree to which caffeine dependence interacts with its sleep–wake effects. Finally, evidence from population-based studies on the role of daily dietary caffeine in disturbed sleep and impaired daytime function will be assessed and the risks of caffeine associated sleep disruption and daytime sleepiness in children and adolescents will be reviewed.
Section snippets
Caffeine pharmacology
Orally ingested caffeine is absorbed rapidly, reaching peak plasma concentrations in 30–75 min.1 It is estimated that 80% of plasma caffeine levels are present in human brain, based on animal studies that have compared plasma to brain concentration.2 Caffeine is metabolized to paraxanthine (80%) and to theobromine and theophylline (16%). With higher caffeine doses, and the repeated consumption typical of regular caffeine users, the plasma levels of paraxanthine accumulate and this paraxanthine
Caffeine effects on sleep in controlled laboratory studies
A number of polysomnographic studies have assessed the sleep effects of caffeine administered within 1 h of sleep. An early study administered 0, 1.1, 2.3, or 4.6 mg/kg (77–322 mg for a 70 kg person) caffeine 30 min before sleep to healthy normals with a reported average daily 3-cup caffeine consumption history.10 Caffeine reduced total sleep time, increased latency to sleep, and reduced percent stage 3–4 sleep in a dose-related manner. REM sleep was not affected.
In a study of the hypnotic effects
Caffeine effects on daytime alertness and performance
Laboratory studies of the effects of caffeine on performance and mood have a long history dating to the late nineteenth century. The acknowledged first placebo controlled study was published in 1907.15 The investigators reported that 500 mg caffeine improved finger muscle strength. The classic review article of Weis and Laties summarized the pre-1960s literature and concluded that the evidence clearly indicates that caffeine enhances a wide range of performance with the exception of
Caffeine dependence
Caffeine dependence is evident by the signs of behavioral and physiological dependence.29 These two dependences often co-exist, but can be differentiated. Physiological dependence is a state induced by repeated drug use that results in a withdrawal syndrome when the drug is discontinued or an antagonist is administered. Among discontinuation effects, withdrawal should be differentiated from rebound phenomenon. Withdrawal is a collection of signs and symptoms that differs from rebound phenomenon
Dietary caffeine in the population: sleep and daytime alertness
Accurate survey data on caffeine consumption in the general population are difficult to collect due to the variety of caffeine sources and the variability of caffeine content in various beverages. A survey of the caffeine content reported by various authors found a range of 64–124 mg reported in a 150 ml (5 oz) cup of brewed coffee.37 This variability is a problem in attempting to associate caffeine consumption with indices of sleep and daytime alertness and requires that investigators assign a
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