The Consequences of Disrupting Biological Rhythms

The Consequences of Disrupting Biological Rhythms
All living organisms experience rhythmic changes, which tend tocoincide with seasonal or daily environmental changes. These rhythmsare known as biological rhythms, which include circadian, ultradianand infradian types.Circadian rhythms are those that repeat themselves daily, thoselasting ?about one day?. The best example of a circadian rhythm is thesleep-wake cycle, associated with which are many cyclical changes withactive and dormant periods, for instance body temperature and urineproduction. These rhythms allow animals to prepare for predictabledaily environmental changes, such as night and day.Ultradian rhythms are those cycles with less than one day. Examplesinclude levels of alertness throughout the day and the cycle of brainactivity during sleep.Infradian rhythms are those with a period of greater than a day.
The menstrual cycle is an example of an infradian rhythm. Infradian rhythms that occur as a result of seasonal changes, for example, migration and hibernation are called circannual rhythms. All biological rhythms are controlled by two different factors - internally (endogenous) through nature, and externally (exogenous) through nurture. Most organisms have internal biological clocks, called endogenous pacemakers. The main endogenous pacemaker in circadian rhythms is the suprachiasmatic nucleus (scn), a small bundle of nerves in the hypothalamus, as suggested by Morgan (1995), and Kalat (1998). Kalat suggested that low levels of light lead to an electrical stimulant, which activates the pineal gland in the scn, located in the centre of the brain, to secrete a hormone called melatonin, which causes sleepiness. Its production of melatonin varies with periods of light and darkness in the environment, and it obtains this information about light in the environment by means of nerve pathways originating in the eyes. He summarised that light slows and darkness stimulates the pineal glands production of melatonin and, therefore, the gland tends to secrete small amounts of melatonin during the day and large amounts at night. The main exogenous zeitgeber (?time-giver?) that controls circadian rhythms is therefore light. Research that suggests the scn is the main endogenous factor for circadian rhythms comes from Morgan in 1995. His aim was to determine whether the scn in hamsters is linked to the disappearance of their circadian rhythms. He removed the scn from hamsters and transplanted an scn from mutant hamsters whose biological rhythms have shorter cycles than those of the recipients. He found that their circadian rhythms had disappeared (when the scn was removed). Further to this he found that the hamsters with the transplanted SCNs took on the mutant rhythms. However, this study can be questioned as the experiment uses hamsters, and it is therefore unreliable to generalise to humans. Wever, who found evidence of a second biological clock, again in the scn, carried out other research. Nearly all the participants in his long-term bunker studies showed evidence of two different patterns - one for their sleep-wake cycle and another for their temperature cycle. This supports the theory that endogenous factors have affect. Research showing that light is the dominant zeitgeber comes from Miles et al. in 1977. He documented problems of a young man who was blind from birth. The subject was exposed to a variety of zeitgebers such as alarm clocks and radios, but results showed that he had a strong 24.9 hour circadian rhythm. The subject had problems resetting his biological clock and had to use stimulants and sedatives to co-ordinate his sleep-wake cycle with the rest of the world. This shows that the absence of light can disrupt biological rhythms and Miles et al. therefore concluded that light is a dominant time-giver. On the other hand, research by Luce & Segal in 1966 showed that people who live in the Arctic Circle still sleep for about 7 hours daily despite the fact that during the summer months, the sun never sets. An explanation for this may be that during this time, other external cues take over, such as social customs for example. Shift work and jet lag have also shown to disrupt biological rhythms. Shift working involves regular changes to the hours of work, it is a feature of our industrialised society. It is a concern because records show that more accidents occur at nights between 1 and 4am or on night shifts, leading to impaired performance and therefore potentially dangerous situations. Frequent shift changes (weekly) are the hardest to readjust to, so longer periods of readjustment (every three weeks/month) are better for the workforce and, consequently, production (Czeisler et al. 1982). Jet lag occurs when flying from east to west or vice versa. It does not occur north-south or vice versa because places to the east are ?ahead? of time and places to the west are ?behind?. When one flies to the west one gains time, but to the east loses time. Klein, Wegman & Hunt, 1972, suggested that adjustments in sleep were faster for west-bound flights. When going eastbound, re-adjustment takes approximately one day per time zone crossed. This is due to the reason that when travelling west, we are ?chasing the sun? and the day is temporarily lengthened (?phase delay?) ? the body prefers this. When travelling east, the day shortens (?phase advance?) ? this shortens the body for what is already a shortened day. The disruptions of the biological rhythms are thought to be caused by melatonin release being out of step with the new environmental conditions. This results in drowsiness during the day and insomnia at night. Evidence from Blakemore, 1988, suggested that this can be treated by the intake of melatonin supplement pills, which can reduce the effect of jet lag.

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