It would be extremely unusual if all people had exactly the same circadian rhythm; some variation in sleep-wake cycles between individuals would be expected in

It would be extremely unusual if all people had exactly the same circadian rhythm; some variation in sleep-wake cycles between individuals would be expected in the same way as a variation in height would be expected. Wikipedia [1] says: "Normal variation in chronotype encompasses sleep–wake cycles that are from about two hours earlier to about two hours later than average." (Unfortunately the citation link is broken.) [1] https://en.wikipedia.org/wiki/Chronotype https://en.wikipedia.org/wiki/Chronotype Also worth reading: [2] https://en.wikipedia.org/wiki/Night_owl_(person) https://en.wikipedia.org/wiki/Night_owl_(person) [3] https://en.wikipedia.org/wiki/Lark_(person) https://en.wikipedia.org/wiki/Lark_(person)