Circadian Rhythms and Sleep
Sabah QuraishiThe word circadian comes from the Latin circa diem meaning about a day, which starts out as a good definition for circadian rhythms. Circadian rhythms do not have to be daily and in fact can happen in multiple periods per day or periods that last longer than days such as circannual rhythms. The most thought of circadian rhythm is sleep but other examples include, body temperature, blood pressure, production of hormones and digestive secretions. These rhythms do not only take place in humans but are present in a wide variety of other organisms, the simplest being cyanobacteria. Circadian rhythms are usually controlled by a combination environmental factors and internal factors. The most common environmental factors are temperature and light.
Circadian rhythms are controlled by a circadian pacemaker, or a biological clock. This "clock" is the section of the brain known as the suprachiasmatic nucleus (SCN). The SCN is a pair of structures that contain about 20, 000 neurons and is located in the hypothalamus above where the optic nerves cross (1) . This proximity to the optic nerve explains its reaction to light. Signals from light are received by the SCN taken in by the retina. Circadian rhythms include three different parts, a central oscillator, afferent pathways that carry environmental information to the oscillator, and efferent pathways that communicate the rhythm of the oscillator to the physiology and behavior of the organism (2) .
A specific example of circadian rhythms, or in this case circannual, is in birds. When it comes time for birds to migrate south and then, back north they are signaled by circannual rhythms. Both the change in temperature and the change in length of day have some affect along with internal signals in signaling birds to migrate. Aside from this there is also a signal that helps the bird navigate. It had been found that the signal for navigation comes from within on first flight, but then is learned over time as the bird continues to fly the path adjusting to its current environment (11) . This shows how a combination of external and internal signals signal a bird to migrate. This is also true of molting in birds.
Cyanobacteria are the simplest known form of life that has circadian rhythms. Using bioluminescent enzymes as markers a gene cluster was found that is involved in the process. This cluster, kaiABC, are proteins that are part of the feedback loop controlling the system (5) . KaiC is involved in setting the clock and depending on the absence of presence of it the clock in cyanobacteria can be changed. The circadian clock in cyanobacteria controls photosynthesis, nitrogen fixation, cell division, respiration, amino acid uptake, and carbohydrate synthesis (6) .
There is a twenty-four hour cycle in drosopholia, or fruit flies. This cycle is controlled by genes per, tim, and dbt which are part of the clock. Clk, cyc, and cry are also clock related genes. The cycle in the fruit fly usually begins at about noon when per, tim, and mRNA levels rise and continues until dawn when per, and tim peak and then decrease (6) . Light is a factor in the timing of the circadian clock in fruit flies and is a part of regulating its behavior.
The most well known circadian rhythm is the human sleep cycle. Sleep is controlled by neurotransmitters, which act on neurons in the brainstem and in the spinal cord (1) . Signals produced by the SCN travel to different regions of the brain. It regulates other functions associated with the sleep cycle such as body temperature, hormone secretion, urine production, and changes in blood pressure (1) . The sleep/wake cycle in humans is dependent on light and temperature. A change in these could shift or disrupt the cycle. External factors that affect the circadian rhythm are called zeitgebers. These could be anything from an alarm clock to meal times.
There are many health problems associated with a disturbance in the sleep circadian rhythm. These can be temporary or due to a lack in the circadian rhythm in the body. These include Seasonal Affective Disorder (SAD) where the rhythm is disturbed due to the change in length of day, delayed sleep phase syndrome (DSPS) which is caused by a circadian rhythm abnormality causing the sufferers body to want to sleep later than normal (3) . More temporary problems include jet lag and problems caused to those working late shifts.
Melatonin is an internal factor affecting the circadian clock. Melatonin is produced by the pineal gland and it has a day/night function. It peaks during darkness and lowers during the day. Melatonin has been shown to shift biological rhythms. There is a correlation with the circadian rhythm so melatonin can be used to shift the rhythm in terms of therapeutic measures. This not only includes human health problems but other cycles such as in sheep to control breeding cycles (7) .
Circadian rhythms seem to fit the profile of brain equals behavior when looked at through the human perspective. The brain takes in inputs along with signals it creates to create an output. What is left unexplained can be seen when thinking backwards. Are circadian rhythms controlled by the brain if organisms without brains posses them? Cyanobacteria lack brains but still respond to circadian rhythms. Aside from cyanobacteria it can be said that plants have a circadian clock which controls photosynthesis and flowering. If this is true what part of the rhythm is controlled by the brain. Could it be that the basic function of circadian rhythms lacks the need of a brain, which is used for only higher functions of rhythms? If this is so where do the rhythms come from.
WWW Sources1) Brain Basics: Understanding Sleep 
3) Your Ticking Clock 
11)Gwinner, "Internal Rhythms in Bird Migration". Scientific American. April, 1986.
Comments made prior to 2007
My 3 year old grandson gets up between 5 - 6 AM in the morning regardless of what time he goes to bed. Any suggestions for resetting his biological body clock ... Judith Cephas, 1 January 2008