[katieHaylor (OP)]

Christine asks:

I have read that the majority of humans have a circadian rhythm that is longer than 24 hours.  If we live on a planet that has a 24 hour day, why would human circadian rhythms be different?  Would our daily exposure to light change our rhythms?

What do you think?

[evan_au]

I have read that the majority of humans have a circadian rhythm that is longer than 24 hours.

I too had heard this.

But apparently the early results of 25 hours was an experimental error - they were not measuring the "free running" circadian rhythm, but the human experimental subjects were allowed access to artificial lighting - and artificial lighting extends the circadian rhythm.

The refined results were very close to 24 hours (but slightly longer, by 10 minutes).

If we live on a planet that has a 24 hour day, why would ... circadian rhythms be different?

Marine animals living near the shore also display a rhythm related to the lunar and tidal cycles, which are longer than 24 hours (by about 40 minutes).

If we live on a planet that has a 24 hour day, why would human circadian rhythms be different?

It is very hard to genetically adjust a biological system to run precisely on a 24 hour cycle.

If the natural cycle time were a bit short or long, we would find ourselves getting further and further out of alignment each day, until we were completely jet-lagged.

Would our daily exposure to light change our rhythms?

Recent studies have found a previously unknown light receptor in the human eye.
It does not produce a conscious image, but does affect the hormones driving the circadian rhythm.

Exposure to natural light synchronises our circadian rhythm to the day/night cycle.

See: https://en.wikipedia.org/wiki/Circadian_rhythm#Biological_clock_in_mammals

[RD]

...  we live on a planet that has a 24 hour day ...

Currently 24 a hour day, Jurassic dinosaurs had a ~23 hour day.
So best to err on a slightly longer day than current, because days are getting longer.

Would our daily exposure to light change our rhythms?

light is the #1 zeitgeber ... https://en.wikipedia.org/wiki/Zeitgeber

[alancalverd]

The mysterious light sensor is exploited by a number of gadgets that add blue light to the immediate environment to reset an individual's circadian rhythm. The objective is to overcome jet lag or Seasonal Affective Disorder. It turns out that the action spectrum for this effect is quite narrow and awkwardly close to the damaging "blue light exposure limit" - I have spent many happy hours evaluating such sources and filters.