Simply put, our bodies have an internal clock called the “circadian.” It comes from the Latin word meaning “about 24 hours.” Our internal circadian “clock” dictates when we feel sleepy, hungry, happy or sad. It is said the human body is actually four-dimensional, the fourth being time, “a daily tide upon which we float.” Most of our circadian rhythms are controlled by our body’s biological “clock.”
How does our circadian “clock” tick?
This clock of ours is actually a pair of pinhead-sized structures that rest in our hypothalamus, which is part of our brain. The hypothalamus rests just above where the optic nerve crosses in our skull. When light reaches our retina (which is behind our eye), it creates signals that travel along our optic nerve to our circadian clock. Signals are then sent along to a gland known as the pineal gland. Our pineal gland responds to darkness and light. The light-induced signal relates to the hormone, melatonin. Our body’s level of melatonin normally increases after darkness, which is why we begin to feel drowsy in the evening. During the night our pineal gland releases the sleep-promoting melatonin. During the day, melatonin stops. This clock of ours also governs functions that are related to our sleep/wake cycle—things like body temperature, change in blood pressure, and hormone secretion.
Delicate balance of cues
Our circadian rhythms can be affected by almost any kind of external time cue: an alarm clock ringing, a baby crying, or the time we eat our meals. Sometimes these rhythms get thrown out of balance; for example, when jet travelers pass through many time zones, they suffer from a disruption to their circadian rhythms and get that uncomfortable feeling of jet lag.
Or I remember my early days of shift work as a staff nurse. My circadian rhythms were disrupted every time I was called onto a night shift and I remember it would take me several days to feel back to “normal” again. Another interesting fact is that people who are blind can experience life-long sleeping problems because their retinas cannot detect light. It is like they suffer from a permanent jet lag. Our rhythms are sensitive to many external factors.