“Morning people” have more caffeine-related sleep problems.
Let me start by saying that I love this caffeine study for personal reasons. As a lifelong night owl, I have been chastised by wife, family, and friends over the years for my regular habit of drinking coffee after 10 pm. (And falling easily asleep two or three hours later, if I choose to.) Other coffee drinkers have told me how rare and weird this is. If we have a cup, they tell me, or even an afternoon sip, we toss and turn all night.
As it turns out, I was talking to the wrong kind of coffee drinkers. I needed to consult my crowd, and that’s what I did. I checked in with a few confirmed fellow night owls, and yes, a few of them reported that they had no problems going to sleep after a late night cup or two.
Anecdotal, of course—but a recent clinical study published in Sleep Medicine backs me up. The study, “Modeling caffeine concentrations with the Stanford Caffeine Questionnaire: Preliminary evidence for an interaction of chronotype with the effects of caffeine on sleep,” sets out to examine the effects of caffeine on the sleep patterns of college students. Researchers at Stanford told the students to keep sleep logs and to wear an actigraphy wristband to record rest/activity cycles. The students filled out daily questionnaires about their caffeine intake at different times of the day, and gave saliva samples for caffeine assessments. The scientists were able to accurately predict salivary caffeine concentrations based on the questionnaires, which was the primary intent of the study. But in the process, they discovered what they believe to be “a novel relationship between the effects of caffeine on sleep and genotype and chronotype.” What the researchers ended up with was some seriously suggestive evidence about the relationship of caffeine and natural sleep rhythms. (Here’s a nifty little test to determine whether you are a lark or an owl, i.e., your chronotype.)
Typically, clinical trials with caffeine are limited to the basic question: How much coffee did you drink today? But the Stanford researchers wanted to include the many variables that modulate caffeine intake—things like the timing of ingestion, the variations in the amount of caffeine among beverages, individual variations in caffeine metabolism, and the wide differences in half-life that caffeine can exhibit under various circumstances. They attempted to establish the students’ genotypes for adenosine receptors, where caffeine does most of its work, and to select volunteers who had “statistically indistinguishable” differences in adenosine receptor gene frequencies.
As you might expect, even among students, caffeine intake progressively decreased throughout the day in the study group. However, a small number of participants continued their intake of caffeine well into the night. The metric known as “wake after sleep onset,” or WASO, was used as the primary measurement of sleep disruption. “Our data indicate caffeine strongly influences WASO in those who self-identify as morning-type,” the researchers found. “It affects WASO less so in those who are neither type, and does not appear to affect WASO in those who are evening-type. To our knowledge, there have been no previous reports linking the effects of caffeine and chronotype.”
Some warnings on the study: It involved only 50 college students. And they were students, meaning their schedules were highly erratic by definition, and they were chronically sleep-deprived by habit. The study authors attempted to turn this defect into a virtue, noting that “the students were under such homeostatic pressure that their mood had little effect on their sleep.” Nonetheless, we will need to see if the findings hold up using less, er, unpredictable subjects.
If they do hold up, it will make it easier for people to understand the homily delivered by the coffee-drinking grandmother of a friend of mine: “The only time coffee ever kept me awake was when I knew there was another cup in the pot.”
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Nova, P., Hernandez, B., Ptolemy, A., & Zeitzer, J. (2012). Modeling caffeine concentrations with the Stanford Caffeine Questionnaire: Preliminary evidence for an interaction of chronotype with the effects of caffeine on sleep Sleep Medicine DOI: 10.1016/j.sleep.2011.11.011
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