The effects of coffee consumption on sleep and melatonin secretion (original) (raw)

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The effects of coffee consumption on sleep and melatonin secretion

Abstract

Background: In this study we examined the effects of caffeine on sleep quality and melatonin secretion. Melatonin is the principal hormone responsible for synchronization of sleep. Melatonin secretion is controlled by neurotransmitters that can be affected by caffeine.

Methods: In the first part of the study, six volunteers drank either decaffeinated or regular coffee in a double-blind fashion on one day, and the alternate beverage 7 days later. Sleep parameters were assessed by actigraphy. In the second part of the study, the subjects again drank either decaffeinated or regular coffee, and they then collected urine every 3 h for quantitation of 6-sulphoxymelatonin (6-SMT), the main metabolite of melatonin in the urine.

Results: We found that drinking regular caffeinated coffee, compared to decaffeinated coffee, caused a decrease in the total amount of sleep and quality of sleep, and an increase in the length of time of sleep induction. Caffeinated coffee caused a decrease in 6-SMT excretion throughout the following night.

Conclusions: The results of our study confirm the widely held belief that coffee consumption interferes with sleep quantity and quality. In addition, we found that the consumption of caffeine decreases 6-SMT excretion. Individuals who suffer from sleep abnormalities should avoid caffeinated coffee during the evening hours.

Introduction

An endogenous clock located in the hypothalamus and affected by the light–dark cycle [1] controls the sleep–wake cycle in humans. The neurohormone melatonin, secreted by the pineal gland, plays a central role in the regulation of this process. Under normal conditions, melatonin levels start to rise about 2

h prior to habitual bedtime, with peak secretory levels usually between 2 and 4 AM, followed by a decreased secretory rate in the early morning [2]. The peak secretion rate of melatonin decreases with age, and it has been shown that treatment with melatonin of older individuals suffering from sleep disorders improves quality of sleep [3].

Melatonin secretion is decreased by pulses of bright light at night, and by a variety of drugs such as beta blockers, benzodiazopines and others [4], [5], [6]. The nocturnal rise of melatonin secretion and the mechanism that causes inhibition of melatonin secretion by several drugs are not completely understood. The complexity of the system suggests more than one mechanism. For example, adenosine affects the accumulation of c-AMP by activating A2 receptors located in the pineal gland. Injection of 5′-_N_-ethylcarboxamidoadenosine (NECA) to rats during the hours of daylight causes a rise in the concentration of melatonin in the pineal gland. Caffeine, a non-specific inhibitor of adenosine receptors, prevents this rise [7], [8].

Section snippets

Aims of the study

We examined the effects of coffee consumption on the rate of melatonin secretion, as reflected by urinary excretion of 6-sulphoxymelatonin (6-SMT), and sleep quality as assessed by actigraphy. The physical movements registered by the actigraph – a small instrument worn like a watch – have been shown to correlate highly with polysomnography in measuring sleep variables [3], [10]. We also determined whether coffee affects the timing of melatonin secretion. Only two studies similar to the present

Methods

The study group was composed of six volunteers who regularly consumed coffee during the afternoon and evening hours. The subjects, three men and three women with a mean age of 32±12 years (mean±SD), were hospital personnel informed of the aims of the study. The study was conducted in a double blind, randomized manner. The subjects consumed either regular coffee (130

mg/cup, pre-prepared portions) or decaffeinated coffee with the same taste from the same company. Subjects were given regular

Results

Subjects were unable to determine whether they had consumed regular or decaffeinated coffee during the study periods. There was no significant difference in their estimation of sleep quality or their reported bedtime.

Statistically significant differences were found in all the sleep parameters examined by actigraphy. Ingestion of regular coffee increased sleep latency and worsened all other sleep variables measured. Sleep parameters are presented in Table 1.

Discussion

In the current work we examined the effect of caffeine consumption during the afternoon and evening hours on various sleep variables and 6-SMT excretion, a measure of melatonin secretion. Caffeine consumption resulted in decreased sleep quality and secretion of melatonin, the principal hormone that regulates sleep. The exact mechanisms for the effect of caffeine on melatonin secretion are not completely clear. One possibility is that caffeine, which acts as an adenosine receptor antagonist [8],

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