Angela
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I don't usually out and out disagree with a paper right from the get go where the authors have done so much research and statistical analysis, but this is an exception. They did something wrong.See: Jorien L. Treurhttps://www.biorxiv.org/content/early/2017/10/07/199828"Study Objectives: Higher caffeine consumption has been linked to poorer sleep and insomnia complaints. We investigated whether these observational associations are the result of genetic risk factors influencing both caffeine consumption and poorer sleep, and/or whether they reflect (possibly bidirectional) causal effects. Methods: Summary-level data were available from genome-wide association studies (GWAS) on caffeine consumption (n=91,462), sleep duration, and chronotype (i.e., being a morning versus an evening person) (both n=128,266), and insomnia complaints (n=113,006). Linkage disequilibrium (LD) score regression was used to calculate genetic correlations, reflecting the extent to which genetic variants influencing caffeine consumption and sleep behaviours overlap. Causal effects were tested with bidirectional, two-sample Mendelian randomization (MR), an instrumental variable approach that utilizes genetic variants robustly associated with an exposure variable as an instrument to test causal effects. Estimates from individual genetic variants were combined using inverse-variance weighted meta-analysis, weighted median regression and MR Egger regression methods. Results: There was no clear evidence for genetic correlation between caffeine consumption and sleep duration (rg=0.000, p=0.998), chronotype (rg=0.086, p=0.192) or insomnia (rg=-0.034, p=0.700). Two-sample Mendelian randomization analyses did not support causal effects from caffeine consumption to sleep behaviours, or the other way around. Conclusions: We found no evidence in support of genetic correlation or causal effects between caffeine consumption and sleep. While caffeine may have acute effects on sleep when taken shortly before habitual bedtime, our findings suggest that a more sustained pattern of high caffeine consumption is likely associated with poorer sleep through shared environmental factors."If I take a cup of coffee at 3PM and try to go to sleep at 10 or 11 I'll be tossing and turning for hours. My results from 23andme gave me a hint as to the reason. I metabolize caffeine very, very slowly. So, I might as well have taken the coffee at 9PM.This highlights one of the problems I see with all these studies on disease and genetics using British Biobank data. Are most of the samples taken from the British Isles? If they are, you cannot assume that what holds true for the British holds true even for other Europeans. In any case, these test subjects were all from Britain. Repeat the test with other Europeans. I've always found it bizarre how on British films and tv series they're making a cup of tea when they're awoken in the middle of the night. For me, that would mean I wouldn't sleep for another 12 hours at least.
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