You don’t hear too many people say, “I’ve just had a cup of coffee, and it’s made me want to sleep.” But it’s true that caffeine makes some people drowsy.
The Internet has plenty of anecdotal evidence for coffee and caffeine’s somnolent effect.
In a forum on Quora.com a woman writes:
I swear to god coffee makes me feel sleepy ~15 minutes after drinking it! It’s soo weird. Im 18 female and im not sure if this affects in some way but yeah, this has always happened! I’ve been drinking it since I was 13 or so. Does this mean I’m tolerant or something?
Matt Burnett submitted a question to TheNakedScientists.com, asking:
Why is it that when I drink coffee it can make me more sleepy? Other people seem to drink coffee and wake up!
Dozens of people writing in the comments on that page reported feeling drowsy from coffee. Some said it was immediate. Some reported feeling drowsy about 30 minutes later.
Ditto. I realized this years ago when I worked odd hours and then again in many years of school. Every time I drink coffee, regardless of the time of day I get drowsy. My mother has had at least three cups a day for years and likewise for my father. However, for me it just makes me want to fall asleep. I even tried drinking it in the mornings consistently for a few weeks. As soon as I stopped drinking it I realized I wasn’t as tired. I assumed I was alone, crazy and it was some paradoxical effect. Nice to know I am not alone!
The people writing in the Naked Scientists comments section may be genetically indisposed to the stimulating effects of caffeine. Some people have a variant of a gene that breaks down caffeine quickly, which makes them alert. Others without the variant apparently do not get the kick right away.
Another reason people get sleepy after caffeine is that people can build up a tolerance for caffeine, a stimulant and the most-consumed mood-altering substance in the world. It’s like alcohol. After months of years of heavy drinking, you need more to get the drug’s full effect.
Yet another reason people feel sleepy soon after taking caffeine is that it may wear off, and, as Men’s Health puts it, “The high comes with a crash.”
“Caffeine is a stimulant, so theoretically, consuming it should increase blood pressure, heart rate and increase alertness and energy; but the effects may be temporary, and once the caffeine is worn off, your fatigue may feel worse,” Kristin Kirkpatrick, R.D., told Men’s Health.
But Livestrong.com published an article that says drowsiness a couple of hours after consuming caffeine is probably because of lack of sleep:
Caffeine can mask your drowsiness symptoms, which can be beneficial if you are tired and need to concentrate on a task, meeting or project. However, caffeine starts to wear off gradually, and when it does, you may find you feel even more fatigued than you were before you consumed caffeine. Remember that caffeine can cover up your drowsiness, but it does not make it go away entirely. If you find you are especially sleepy a few hours after consuming caffeine, this has more to do with your lack of sleep than it does the caffeine you consumed.
Also, says Men’s Health, caffeine is a diuretic—it makes you urinate a lot, which may dehydrate you. Dehydration too can make you feel sleepy. Men’s Health says you have to consume a lot of caffeine to get the diuretic effect, however.
All those people writing in to the Naked Scientists were giving anecdotes, which are not considered valid scientific evidence.
In fact, most people who consume caffeine are stimulated by it and feel more awake. Dr. Gary Wenk, a psychologist posting on ShareCare.com, wrote:
There are two major effects depending upon the dose of caffeine and how tolerant you are to its effects upon your body. First, a typical cup of coffee will double the time it takes you to fall asleep. This is called your sleep latency. Second, coffee will reduce the amount of deep sleep you get and induce more nighttime awakenings. This is unfortunate because you need those deeper stages of sleep for healing and memory consolidation. In contrast, caffeine has no significant effects upon the amount of REM or dream sleep you get each night.
Some experts advise people should stop drinking coffee or taking caffeine in other forms about 8 hours before bedtime because of caffeine’s half life of 4 hours. This means it takes 4 hours for half the caffeine in your system to dissipate, and another 4 hours for half of that caffeine to disappear.
Studies have found that caffeine consumers could benefit from abstaining. CoffeeandHealth.org reports:
A 2008 systematic review, including randomised trials, suggests that caffeine abstinence for a whole day could improve sleep quality and could be recommended by health practitioners when giving sleep hygiene advice.
The scientists apparently haven’t found too many people who doze off after coffee. Maybe they’re a rare breed.
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