So the "health" section of the WSJ needed to come up with a story about sleep, and found that many things correlate with the number 7. The stone age-like study

So the "health" section of the WSJ needed to come up with a story about sleep, and found that many things correlate with the number 7. The stone age-like study mentioned at the end, for example, simply found that their participants slept longer when they had no access to electricity: "subjective assessments of health and functioning did not reveal any relevant changes across the study." So why 7.2 hours? Sunset at 22:00, sunrise (or excessively loud birds!) at 5:30... Even the title seems wrong. Sleep cycles take about 90 minutes each, so if I plan to wake up after a certain amount of time, I pick a multiple of 90 minutes, i.e. neither 7 nor 8 hours.