Biological Time
Time is usually considered as something that happens to us. It's measured externally - by the ticking of the clock, the movement of the sun, the passing of the seasons. However time is also marked by our bodies at the most fundamental level. The study of this relationship between living organisms and time is known as chronobiology.
Body Clocks
Circadian Rhythms
Most people are probably familiar with the term circadian rhythm. It refers to the body's daily cycle and is usually associated with the sleep cycle. If our daily rhythm is interrupted then we can get "out of synch". A temporary exmple is jet lag. Long term conditions include Advanced Sleep Phase Syndrome (ASPS).What is less well known is that the body has not just one but many different circadian rhythms. Researchers have identified some 100 different rhythms in our bodies, including blood pressure and body temperature as well as the sleep-wake cycle. These various rhythms interact, hopefully meshing together correctly.
As well as circadian (daily) rhythms, we also have longer term rhythms (infradian rhythms - eg the menstrual cycle) and shorter term ones (ultradian rhythms - eg the NREM-REM cycle during sleep).
How are these rhythms controlled? A large part of it is clearly external. For example, the presence or absence of sunlight has an effect on the level of bodily hormone generation. These hormones in turn regulate some of our circadian rhythms.
It turns out that our body also has a highly developed "master clock" sitting in the brain.
The Suprachiasmatic Nucleus
The suprachiasmatic nucleus (SCN) is an organ about the size of a pea found in the hypothalamus. It's not restricted just to humans.THE SCN is in effect our "master clock". Although it uses external signals - such as daylight - to regulate itself it's essentially self-contained. A gene/protein expression cycle in the SCN generates the 24 hour cycle of neuronal activity.
The SCN interacts with various other portions of the brain to produce circadian rhythms, core to which is a 24-hour rhythm of neuronal action potential. This action potential reaches maximum around midday and falls off during the night when we are (supposedly) asleep.
So within the SCN we each have our own highly advanced molecular clock, a biological masterpiece more advanced than any horologist could conceive.
It's interesting to speculate about the effects of the SCN on the development of our minds. Is an awareness of time necessary for consciousness? Is a simulated SCN essential for true Artifical Intelligence? Could the SCN prove to be the basis for the psychological arrow of time?