Caffeine is a well known and widely consumed stimulant as most adults will start the morning with a fresh cup of Joe. Although caffeine is more commonly ingested in the form of coffee and tea, caffeine can also be found in a number of other substances, including soft drinks, energy drinks, chocolate, supplements and some medications.
The impact of caffeine on our sleep-wake rhythm comes from action as an adenosine receptor antagonist. As such, it binds to the adenosine receptors in the brain, blocking their action and preventing proper signaling of adenosine levels. But what is adenosine you may ask? Adenosine is a by-product of energy expenditure from our cells. Thus, an increase in energy expenditure will result in increased adenosine concentration in our body.
Adenosine levels have an inhibiting response on the wake promoting system in the brain. Therefore, the gradual increase in adenosine levels throughout the day promotes sleepiness. Adenosine levels are consequently related to the homeostatic pressure for sleep, one of the two main processes in sleep-wake regulation. As caffeine suppresses the action of adenosine on their receptors, it reduces the pressure for sleep and promotes wakefulness.
The homeostatic pressure for sleep has influence on the structure of our night-time sleep, particularly on the duration and intensity of our deep slow wave sleep. This sleep stage has a important role in recovery from our daily activities, as well as in the formation and consolidation of memories. Consequently, in addition to experiencing difficulties falling asleep and potentially more awakenings throughout the night, caffeine also interferes with the proportion of slow wave sleep during the night. As a result, the impact of caffeine on the quantity and quality of our sleep can lead us to feel even more tired the following day and, therefore, requiring more coffee…
Nevertheless, although caffeine has been shown to influence our ability to fall asleep, we all know at least one person who can drink an extra large coffee at 10 pm at night and be asleep within the next hour. Interestingly, the effects and intensity of caffeine is highly variable from one individual to the next. Sensitivity to caffeine is, in part, based on your genetics. Variants in the adenosine receptors will make some people sensitive to caffeine in their system, as for others have no problems falling asleep after a cup of coffee.
Changes in sensitivity are also dependent on various situations and circumstances. For example, nicotine has been shown to increase the metabolism of caffeine. On the other hand, pregnancy and use of oral contraceptive can slow down its metabolism, prolonging its effect.
Overall, and depending on the method of consumption, effects of caffeine are felt about an hour after ingestion. Caffeine has an average half-life of 3.5 to 4 hours, meaning that it take such time for half of the ingested dose to caffeine to be metabolized by the liver. However, this can span from anywhere between 2 and 10 hours, due to individual differences as some digest caffeine faster than others. The speed of digestion becomes particularly important when caffeine is consumed in the afternoon or evening, as remnants of caffeine can interfere with night-time sleep.
In the end, it is important to find what works best for you as sensitivity varies from one individual to another. Nevertheless, if you are experiencing difficulties falling asleep at night, moderating your caffeine intake, and more particularly after lunch, may help your body feel sleep come bedtime.
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