Response to Dana's suggestion on tidal-circadian rhythms:
PT: This is outright nonsense. Rhythms are governed by dark/light cycles and the rhythm can be variable. Consider beavers, an actual aquatic mammal (as opposed to terrestrial species like humans):
http://www.fs.fed.us/r2/projects/scp...icanbeaver.pdf
North American Beaver (Castor canadensis): A Technical Conservation Assessment
...
In winter, beavers may display a circadian rhythm of 26 to 28
hours when living under pond ice in the shelter of the lodge (Wilson and Ruff 1999). ...
The 24 hour 50 minute figure for humans is the free running, i.e., UNregulated, period. Humans today, much less our ancestors, do not live in an unregulated world. We go by the night day alternation (which is actually a few minutes under 24 hours). Modern living - economics, jobs etc - seriously disrupt sleep cycles. The real fatal flaw in this rubbish follows from this:
"It occurred to me that an aquatic ape living by the sea would fall into that rhythm, presumably to be ready at each low tide to gather mussels, clams, crabs, etc." (Dana M.)
Here's a tide schedule for the city of Seaford in the state of Virginia:
Tide schedule
Times for high and low tides vary quite a bit. They do not occur at the same time each day. Consequently, we could not time out circadian rhythm to coincide with tidal schedules.
DM: Of course the workaday circadian rhythm of modern humans is governed by light/dark and myriad other cues. No argument there. It's the rhythm
in the absence of those cues that intrigues. Without those cues humans seem to fall into a daily sleep/wake cycle of ~25 hours. Interestingly, even with all of the cues many people still feel like they should be on a 25 hour cycle, that it's more natural for them.
I merely suggest that this apparently natural human rhythm conforms to a "daily" tidal cycle, which includes two high and two low tides. The very website for the tides of Seaford, Virginia that Mr. Travsky
employed to refute my argument actually confirms it. Look at the time of any tide. The next tide of the same type (high or low) will be 12 hours and ~25 minutes later, and the next after that will be 24
hours and ~50 minutes later, creating a tidal "day" of 24 hrs 50 min. That pattern, with few variations, occurs the world over, has done so for eons, and is suspiciously similar to our natural circadian rhythm of ~25 hours.
Mr. Travsky's beaver example is interesting, but not directly applicable. Beavers are creatures of lakes and streams which do not exhibit tidal variation. The sea coast does. And that tidal rise and fall would have been of critical importance for any human ancestor trying to gather food or perform other activities. A sleep/wake cycle that evolved to conform with the tides would make sense.
My argument is not against the obvious fact that modern humans are regulated by the sun and the clock and not the sea, but rather that we apparently retain vestiges of a circadian rhythm from a distant
past that was lived on a sea coast, a daily cycle that was then regulated by the tides.
MV: Yes, excellent idea, but we first have to know what are the diurnal rhythms in other animals incl.sea-otters, coastal otters (Aonyx) etc. If it is as Dana thinks, our littoral phase seems to have lasted until *very* recently, perhaps even after the sapiens LCA.
DD: I tend to agree with Dana, that humans have maintained a tidal influence in our behaviour, although we mostly lost the seashore daily diving/foraging long ago.
Here's something on circadian rhythm disruption due to daylight saving time change of one hour:
A Blog Around The Clock : Daylight Savings Time worse than previously thought
DDeden
