The Human Circadian Rhythm
A TV documentary on the human perception of time (UK BBC2 "What makes us tick?" 9th Jan 1999) mentioned that the human circadian rhythm is generally about 24 hours 30 minutes.
Normally, the drift between our internal clock and the actual length of an Earth day is reset each morning by the rising of the Sun. Deprived of this trigger a test subject, who lived deep in a cave for 6 months in the late 60's, was reported to have experienced a gradual shift in his usual pattern such that his day lengthened to over 24 hours.
Also, it was reported that some blind people suffer from sleeping difficulty due to their isolation from the reset trigger of light and dark. One individual kept a diary of his sleeping times, and noticed that he experienced an internally-clocked day length of 24 hours and 40 minutes.
I started to wonder about this.
Earth's sideral day is 23 hours 56 minutes and a few seconds. Why aren't our internal clocks synchronised with the day length on our own home planet?
The Earth's rate of rotation is slowing down - that's why we have to have leap seconds every so often, to keep our civil calendar in sync with our natural cycle.
As the Earth's day is currently shorter than our internal day, we can't ascribe the discrepancy to an evolutionary remnant of a time when Earth's day matched our own. The discrepancy is in the opposite direction - the Earth would have had to have had a slower rotation in the past, not a faster one.
Out of curiosity I looked up the sidereal day length for Mars... it's 24 hours, 37 minutes and 22.8 seconds.
[thoughtlink: Paul Davies - did life originate on Mars?]