(Drawing courtesy of Yassine Mrabet) A bio clock typical of someone who rises early in the morning, eats lunch around noon and sleeps at night. Information from "The Body Clock Guide to better Health. http://www.amazon.com/Body-Clock-Guide-Better-Health/dp/0805056629/ref=sr_1_1?ie=UTF8&s=books&qid=1254868218&sr=1-1 "
What Makes Us Tick?
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NOVEMBER 03, 2009
April Holladay, HappyNews Columnist

Q: When we sleep on a bus trip, why are we often able to wake up just before our intended stop?
Joe, Singapore
A: You share an ability with creatures large and small. Bats wake up to hunt because their clock tells them it's dusk - summer or winter, no matter. A white-crowned sparrow fattens for weeks ahead of migration time because his clock triggers. A honeybee knows the exact time to visit a particular flower - when that flower is making nectar. Her "alarm" goes off and she flies to that flower to sip the sweet liquid.
Biological clocks exist and you have one. Why you can set your alarm clock and wake up when you want takes us to the frontiers of knowledge. We barely understand how the clock works. According to experts, no study has rigorously investigated your question. So, I cannot answer your question completely but I can explain how bio clocks work and discuss an alarm system that bees learn. Perhaps your alarm system evolved from a similar need.
Worker bees have no daily rhythm when they first emerge from their cocoons as adults. They work in the hive tending the queen, keeping house, and caring for the brood: tasks that busy them around the clock. Later, they emerge from the hive and forage for food.
Forager bees live or die by the clock. They must know when a flower opens shop and offers its nectar. They learn to use a clock. Out a bee goes. When she finds a flower that's open and she gathers its nectar, she records the place and time: say, 11 a.m. The next day at 11 a.m., a nerve synapse sends a message to the bee's brain and the bee returns to that flower and gets its nectar. Otherwise, if the bee gets no nectar from a flower, she makes no entry in her brain. In this fashion, the new forager gradually builds a flower diary and makes her daily rounds accordingly.
Animals use their clocks so many ways it's difficult to see how they could survive without such a mechanism. Our common clock reflects either a common evolutionary ancestor or a clock that arose from the same overpowering need but along several evolutionary paths.
Whatever the source, almost all animals have bio clocks and they work much in the same way. In humans, the clock is a system with three primary elements: a clump of brain cells (called the SCN), light receptors in the eye that talk to the SCN, and a gland that makes hormones when the SCN says to.
More exploring:
How the SCN, eye light receptors, and a hormone gland tell time, WonderQuest
Readers' Answers:
• In my case (male, 86, with enlarged prostate) it is the all too familiar "latch key" rush. I am sure my body has a sensing device that tells me the buss is due for a stop and that I must be ready to exit promptly and single-purposedly! :) But FAST!
Chuck, Fort Wayne, Indiana, USA
• When we go to sleep, we have a good idea as to how long the trip should take. Our internal clock uses this information and wakes us up in time to make our stop.
George B, Crownsville, Maryland, USA
• I think it is because our brain is preconditioned with the approximate time that will elapse from the start of the journey to its end. The alarm so set in our brain sends out the signal to wake up.
Jose, Eraviperoor, Kerala, India
• Usually, as we sleep, our bodies become limp and are easily moved around. And, as we all know, Bus Drivers are dangerous drivers. After braking very quickly, everyone that's asleep whacks their heads on the seat in front, waking everyone up.
Josh, Southport, United Kingdom
• You are able to wake up because the body does not go into REM sleep and responds to clues like the bus slowing down, swaying of familiar turns, light and dark through eyelids, people moving around you and perhaps even time range of when you expect to arrive.
Jean, Canada
• I believe: (1) you have a basic idea, of how long the trip will be, so your "internal" clock wakes you, at the approximate time, and (2 ) the slowing and stopping, especially after a long trip, of the buss, awakens you.
Robert Westerholm, Phoenix, Arizona, USA
• Presumably, most of the trip was highway driving at near constant speeds. Closer to home, you will slow down and speed up again, stop at red lights, etc, which will usually wake up a sleeping person.
Ellen, Mountain Brook, Alabama, USA
Further Reading:
- PBS, Nature: how a bee's brain senses time
- The Food-Energy Cellular Connection Revealed: Metabolic Master Switch Sets the Biological Clock in Body Tissues, University of Michigan, October 2009
- Discovery About Biological Clocks Overturns Long-held Theory, Salk Institute for Biological Studies, October 2009
- Monitoring Circadian Rhythms of Individual Honey Bees in a Social Environment Reveals Social Influences on Postembryonic Ontogeny of Activity Rhythms, The Hebrew University of Jerusalem, Jerusalem, Israel, Journal of Biological Rhythms, Vol. 22, No. 4, 343-355 (2007)
- The influence of time of day on the foraging behavior of the honeybee, Apis mellifera by Moore D, Siegfried D, Wilson R, and Rankin MA (1989). Journal of Biological Rhythms 4: 305- 325.
(Answered 9 Nov. 2009)