Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: davekm on 19/01/2014 17:22:16
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I have checked the sunset and rise for my city of residence, Sheffield, UK. It challenges the assertion that you need 8 hrs of sleep in the summer. There is only a 7 hour gap from sunset to sunrise in June, a 7 1/2 hour gap in July and a 8 hour 6 min gap in May. Our circadian rhythm evolved from homo sapiens who slept when it became dark and were awakened by sunlight. Furthermore, it stays light for an hour after sunset. So, in the UK if you slept an hour after sundown in June, you will only get six hours sleep before sun rise at 4:30am. This put's pay to the belief that humans need 8 hours of sleep. Likewise, there is a 16 1/2 hr period till sunrise in December, suggesting we should almost hibernate in the winter.
I guess I really need to know the sunrise and sunset that we evolved to. I know homo-sapiens have the PER3 gene, I don't know if homo-erectus did though. If our biological clock was evolved from homo-sapiens that would mean we followed the sunset hours of Africa. I'm not sure what those sunset hours were, as the equator has moved as much as 110 km over the past million years. On top of that, the continents move about 20 kilometers per million years, some places move 70 kilometers per year. So I doubt I could predict past sunlight hours based on modern equatorial Africa, which is around 12 hours of sun daily.
Here is the sunset and sunrise in Sheffield UK in the middle of the month -
Rise Set Hours of Daylight
Jan 8:13 16:17 8h 04m
Feb 7:26 17:14 9h 48
Mar 6:20 18:10 11h 49
Apr 6:06 20:06 14h
May 5:05 20:59 15h 54m
Jun 4:36 21:36 16h 59
Jul 4:56 21:26 16h 30
Aug 5:45 20:33 14h 48
Sep 6:39 19:21 12h 41m
Oct 7:32 18;09 10h 36
Nov 7:31 16:09 8h 37
Dec 8:15 3:46 7h 31
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"Circadian" refers to the whole cycle of activity when unprompted by light or other time cues. By experiment it turns out that this is about 25 hours. Within that time frame, different species have different natural sleeping patterns and when prompted by daylight, different distributions of sleep and activity around the daylight period.
Since a single species does not vary genetically with latitude or season, it would be surprising if the optimum sleep period for a large mammal like a human was to depend on daylight hours, though it might for small animals like mice which are active at dawn and dusk and for large animals (e.g. bears) whose food availability is strongly seasonal. And thus it turns out.
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I don't think you can say that the shortest night in Sheffield proves that the average human needs less than 8 hours sleep. Otherwise, you would look at the Inuit living in the Arctic, and say that their shortest night is zero minutes, so nobody ever needs to sleep!
Hormonal levels and activity levels vary with the seasons. It could be that during long hot summer days, people tend to sleep less at night, and have a siesta after lunch...
Candles, electric light and television (and now internet) have made great changes to the amount that people sleep. A normal distribution suggests that some people aren't getting enough of it, while others are getting more than they strictly need.
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Your both right. All animals and plants possess a circadian rhythm, so I suspect the circadian gene is much older then the dawn of homo-sapiens. Some research claims our ancestors slept in segmented periods (4 hours wake, 4 hours later or 6 hours wake, 2 hour siesta). A researcher named Roger Ekirch is prominent in researching segmented sleep. http://en.wikipedia.org/wiki/Segmented_sleep
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In Malcolm Gladwell’s book The Outliers, which is about the influence of culture, opportunity, and experience on achievement, he mentions that European peasants worked very hard between late spring and early autumn, but did little in the winter except hunt a bit, and feed the fire. He quotes historian Graham Robb who claims: “After making the necessary repairs to their tools, these vigorous men will now spend their days in bed, packing their bodies tightly together in order to stay warm and eat less food. They weaken themselves deliberately...Human hibernation was an economic necessity. Lowering the metabolic rate prevented hunger from exhausting supplies.”
It made me wonder if Season Affective disorder is any more common in people of northern Europe descent, and if what seems like a form of depression to us was once an adaptation.
I think many people do find it is harder to get up at 5:30 in the morning in January than June.
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A new word to me - "Chronotype": Whether you prefer to get up early, or sleep in.
30 minute podcast: http://www.bbc.co.uk/programmes/p01prctn
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European peasants worked very hard between late spring and early autumn, but did little in the winter except hunt a bit, and feed the fire.
Interesting. One of my clients is a chiropractor in rural Cornwall (the bit of England nearest to Canada, for our transatlantic members). I asked him how many x-rays he takes in an average month. He replied "There isn't an average month. These blokes work 18 hours a day, in the fields and woods in summer, or lambing, feeding animals, and repairing machines and buildings in the winter. Except in July when the animals are out grazing, the crops are growing all by themselves, and every farmer says 'bloody hell, my back hurts!' "