A term with two significantly different meanings. Both meanings deal with the cycle of life, though at vastly different granularity.
Circadian Rhythms
When speaking of the routine of daily life, and how we live it, biological clock is a plain-language synonym for the more scientific term circadian rhythms. These rhythms act as the body's "event timer", regulating wakefulness, body temperature, blood pressure, hormones, and even behaviour. In humans, a region in the hypothalamus called the
Suprachiasmatic Nucleus controls our body clock. Animals, plants, and even fungi have circadian rhythms, though the mechanisms that produce them are very different.
Variations in light levels help us to synchronize our circadian rhythms with the Earth's day/night cycle, keeping our body clocks accurate. Research has found that without day/night cues, our body clocks can drift, with different body functions falling out of synch with each other.
Jet lag may be partly caused by the brain's attempt to resynch its clock with the external signals it receives. This may also help to explain difficulties such as SAD, where a lack of natural light in the winter months allows the body's clock to drift out of proper phase, causing depression and moodiness.
Melatonin, a hormone secreted by the pineal gland, has been found to help stabilize circadian rhythms in people whose cycle is unstable, which may explain its popularity and sometime-success as a sleep aid.
Related body cycles, which are less well documented and more speculative, include the circannual rhythm - an annual cycle, and the circaseptan rhythm - a 7 day cycle thought to impact fever, immune-related response, and transplant rejection amongst other phenomenon. These too may be part of our physiology's system of biological clocks.
Diminished Ovarian Reserve
Much more emotionally charged is the other common meaning of biological clock -- the amount of time a woman who desires pregnancy and childbirth has to achieve those goals before her body becomes unable to accommodate them.
Men, especially single men, often don't understand the issues involved. They may think it's a kind of joke, having to do with marriage and some sort of female peer pressure. The biology of it is not something men are commonly aware of, at least not until their female partner begins to worry about it. After all, Father Time has little effect on male fertility. Males produce sperm throughout their adult life, and are quite able to father children with much younger partners.
Mother Nature is not so kind to her half of the gender divide. A woman is born with her total life supply of eggs. Unlike sperm, no more eggs are made by the body after birth. Once a woman's menstrual cycle begins in her early teens, she starts to use up her supply, on a more-or-less monthly basis, until things start to slow down in the mid-40s or so and finally stop with menopause. And of course there's no way to tell how many eggs life's lottery gave you, or are left in the bank at any given time. So, the biological clock is a sort of 'countdown clock' to the end of the possibility of pregnancy -- but there's more to it than that.
Not only does the supply run out, but as time runs on, the viability of the remaining eggs decreases. Remember, the eggs are all there at birth. As a woman ages, the chance of chromosomal injury to the remaining eggs increases as well. Ovulation of the healthiest and most fertile eggs occurs in a woman's teens through her late 20s, ironically when she is least likely to want to use them in the Western world. By her mid to late 30s the remaining eggs are less fertile, and an increasing percentage of the remaining eggs are likely to be abnormal.
If these chromosomally abnormal eggs become fertilized, defective embryos result. This seldom produces healthy pregnancy. When a defective embryo implants in the uterine lining, the body will often detect the abnormality and miscarry during the first trimester. Often this will occur soon after conception, seeming like a slightly late and heavy period. More crushingly it may happen after the woman has determined that she is pregnant, and begun planning for the baby's future.
Even if conception is successful, the risk of other health complications increases for mother and baby both as the woman ages. Among the risks are Anencephaly, Spina bifida, and chromosomal abnormalities such as Down syndrome.
In the digital age, the term biological clock leads us to a false impression of the true import of the situation for a woman who desires pregnancy. We're used to seeing digital doomsday clocks in spy films, helpfully telling the hero(ine) exactly how many seconds are left to defeat the villainous plot of the moment. This clock is not so precise. It's more like an hourglass, glimpsed only dimly but casting a long shadow over the hearts of couples who met later in life or left their familial ambitions too long.
Sources included:
- http://www.healthy.net/scr/column.asp?ColumnId=1&ID=362
- http://www.wrtl.org/PregnacyAfter35.html#01
- http://www.caresmed.com/education/age_and_fertility.html
- Living through the ongoing saga, of which more soon.