Within two hours of having swallowed a cup of coffee, the concentration of caffeine will hit a peak in the brain, giving your mood a buzz and helping your muscles feel less heavy for a bit longer. It also constricts blood vessels in the brain. By allowing certain nerves in the brain to continue firing at a relatively rapid pace, we're in effect telling other circuits — like those linked with the pituitary gland — that something exciting is happening.
It responds by releasing epinephrine, a hormone that generates responses we might need in times of stress or danger. Our heart beats a little faster, pupils dilate, blood is diverted to our muscles, and stored carbohydrates are released from the liver. Generally speaking, most people with a habit of consuming the equivalent of one to four cups of coffee a day — around milligrams of caffeine in a 24 hour period — face no clear risks to their health.
In fact, a small amount of caffeine might even have a few small , but not insignificant benefits, including preventing heart disease , helping muscles burn calories , and possibly even boosting your chances of living longer. The maximum effects of caffeine usually occur between minutes within consumption, although this timing can vary widely among individuals.
After being absorbed, caffeine is efficiently distributed throughout the whole body, and it crosses the blood-brain barrier. Inside the brain, caffeine blocks adenosine receptors. Adenosine is a sleep-promoting chemical that is produced in the brain during our waking hours. The more it builds up, the sleepier we become. When caffeine blocks this process, we remain alert and vigilant. Research has also shown that caffeine interferes with circadian melatonin rhythms , delaying the onset of sleep if consumed close to bedtime.
Circadian rhythms are physiological patterns, like our sleep-wake cycle, that operate on a hour clock. They are held in check by the external cycle of day and night, and by internal cellular processes. The effects of caffeine are measured by its half-life , which typically ranges from hours.
The half-life means the time that your body has metabolized, or processed through, half the caffeine you consumed. As a result, the six-hour half-life of a caffeinated beverage you consume in the afternoon could keep you up at night.
Some factors can slow or speed up the metabolism of caffeine. By contrast, pregnant women experience slower caffeine metabolism.
It is recommended that those who are pregnant or breastfeeding should avoid coffee , or limit intake to 16 ounces or less per day about two cups of coffee. During pregnancy, caffeine can be passed to the baby through the placenta.
A trace amount of caffeine can also be found in breast milk. If you are pregnant or breastfeeding and concerned about your caffeine intake, talk to your health care provider about what steps you should take to ensure optimum health for you and your baby.
Caffeine can impact the onset of sleep and reduce sleep time, efficiency, and satisfaction levels. Older adults may also be more susceptible to caffeine-induced sleep troubles. Caffeine notably reduces the time of slow-wave sleep , which is the stage of deep, restful sleep that leaves us feeling refreshed and alert in the morning. Caffeine-interrupted sleep can lead to sleep deprivation the following day, which is characterized by fatigue and problems with learning, memory, problem-solving, and emotion regulation.
One study examined the effects of caffeine intake zero, three, and six hours before bedtime, and found that even caffeine consumed six hours before bed could reduce sleep time by one hour. In addition, study participants reported sleeping problems when consuming caffeine hours before bed, but they did not realize their sleep was also disrupted when consuming caffeine six hours before bed. If you have difficulty sleeping, consider limiting any caffeine intake six hours before bed.
Caffeine can cause a burst of energy as it stimulates the central nervous system. Most people drink coffee in the morning to help them wake up, but caffeine consumed at night can cause more harm than good. While caffeine can boost cognitive function in the severely fatigued, it cannot permanently ward off sleep or fix the effects of long-term sleep loss. Even though caffeine can marginally boost performance, it is no substitute for a restful, restorative night of sleep.
It is also proposed that the effectiveness of caffeine varies depending on the dose and the state of the individual. For example, the arousing effects of caffeine may benefit someone who feels foggy and tired. For someone who is already alert and aroused, however, caffeine may cause over-arousal and lead to anxiety, restlessness, and dependency. While there are short-term performance benefits to caffeine consumption, overuse can lead to insomnia symptoms or worsen pre-existing insomnia.
Consuming caffeine to stay awake at night may lead to sleeplessness, anxiety, frequent nighttime awakenings, and overall poorer sleep quality. In addition, caffeine from sodas has been linked to increased severity of sleep-disordered breathing SDB , which is the primary characteristic of Obstructive Sleep Apnea OSA. Notably, this association was only found with caffeinated sodas, but not with coffee or tea, although it is unclear why.
Regardless, those who suffer from sleep-disordered breathing may not know about their sleep disruption and find themselves attributing excessive daytime sleepiness to other causes. If you struggle with sleepiness during the day, it might be helpful to reevaluate your caffeine consumption, especially from sodas. It may seem counterintuitive, but some people feel like caffeine makes them tired instead of more alert.
The disruptive effects of regular caffeine use on sleep can create a vicious cycle. Caffeine use causes sleep deprivation, and sleep deprivation causes sleepiness the subsequent day, which in turn causes an increased need to consume more caffeine in order to cope with the sleepiness. Expectation of having consumed caffeine can improve performance and mood. Appetite , 57 3 , — James, J. Effects of caffeine on performance and mood: withdrawal reversal is the most plausible explanation. Psychopharmacology , 1 , 1—8.
Jin, M. Korean journal of family medicine , 37 2 , — Frontiers in pharmacology , 9 , McLellan, T. Adrenaline is the " fight or flight " hormone, and it has a number of effects on your body:.
This explains why, after consuming a big cup of coffee, your hands get cold, your muscles grow tense, you feel excited and your heart beats faster. Adenosine isn't the only neurotransmitter affected by caffeine.
Read on to learn about how the drug affects dopamine, another important chemical in the body. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. Prev NEXT.
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