Tick Tock, Internal Clock

When the alarm clock rings in the morning, some of us wake up and feel an overwhelming urge to hit the snooze button and go back to sleep. Barely able to drag ourselves out of bed, we hear the cheerful “good mornings” of others, and secretly wonder how they can be alert and fully conscious at such an early hour. Conversely, there are those of us who love to begin each day by bouncing out of bed and throwing open the curtains, but cannot understand the people who merely cringe and retreat deeper into their blankets. The affinity of these early-rising “larks” and late- waking “owls” for either morning or evening is more than a matter of personal preference. The reason for such differ- ent behaviors is biological, caused by slight differences in the internal clock. What is an Internal Clock? An internal clock is a commonly used term that refers to the body’s ability to maintain an inner rhythm. Physically, the clock is composed of two small, olive- shaped clusters located at the base of the brain, in the ven- tral hypothalamus region. This pair, each containing about 10,000 neurons, is referred to as the suprachiasmatic nucle- us, or SCN (Koukkari and Sothern 2006). The SCN is pri- marily responsible for overseeing the body’s circadian rhythms, which are biological processes that repeat every 24 hours (Dunlap 2004). Humans typically spend a 24- hour day in alternating periods of sleep and wakefulness. An important function of the clock is to maintain these daily sleep-wake cycles (Horne 2006). More specifically, the clock controls when feelings of alertness or sleepiness are most acute. Due to the internal clock, a person will experience a 24 hour rhythm of changing alertness levels, similar to that shown below: The Effects of Circadian Rhythms on Human Sleep-Wake Cycles by Elaine Tu Tick Tock, Internal Clock THE MIND FALL THE MIND Notice that there is a dip near mid-afternoon. The presence of this small decrease in alertness is a possible explanation of why some people feel the urge to nap after lunch. With the exception of the dip, the rest of the chart matches the expected human activities, with high levels of alertness cor- responding to times spent awake and low levels of alertness corresponding to times spent sleeping. Experimental results have confirmed that, much like a mechanical clock, the internal clock can keep time by main- taining a rhythm independent of the environment. Internal Timekeeping To uphold a periodic sleep-wake cycle, the body must have a method of keeping time. Experimental results have confirmed that, much like a mechanical clock, the internal clock can keep time by maintaining a rhythm inde- pendent of the environment (Koukkari and Sothern 2006). In order to test the self-sustaining nature of the internal clock, scientists have developed a series of “free running” experiments, in which the body is allowed to run its natural rhythm without the presence of external time cues. One such study, conducted on 11 young men and 13 older sub- jects, showed that the period of the circadian oscillator was 24.18 hours (Czeisler, et al.). Indeed, although the exact times may differ, all free running experiments have pro- duced results which suggest that circadian rhythms have an endogenous period that is slightly longer than 24 hours (Refinetti 1999). In addition to the period of the SCN as a whole, rhythmicity also exists at the cellular level. By cutting away a rat’s SCN and observing it in a laboratory dish, researchers found that individual SCN cells retained a rhythm with a period different from that of the entire SCN. In one experiment, the scientists concluded that the mean circadian period was about 24.2 hours, while the period of each cell ranged from 20 to 28.3 hours (Honma et al.). The ability of the SCN and individual SCN neurons to keep time independently implies that the period is genetically determined, and will continue in the absence of external time factors (Koukkari and Sothern 2006). Even so, Courtesy of: Horne, Jim. 2006. Sleepfaring. Oxford: Oxford University Press.