and even social interaction and help the biological clock maintain a 24-hour day.
There are many factors influencing biological rhythms. A cluster of approximately 10 000 nerve cells located on the suprachiasmatic nuclei (SCN) found on the hypothalamus in the brain. The circadian clock's primary function is to interpret external changes of light and darkness, as well as social contact, in order to establish diurnal rhythms. It is not uncommon for the circadian clock to be disrupted temporarily; events such as changes in work schedule from day to night, changing time zones, and to some extent old age can impact the consistency of circadian rhythms.
The circadian clock relies heavily on changes in light to determine day/night transitions. During the night, SCN emits melatonin hormone, which induces sleep. The process of wake to sleep itself has its own stages and each stage has its own duration [17].
Another major disruptive factor related to the circadian clock's interpretation of light is seasonal change. During the winter months, there are fewer daylight hours. As a result, the level of melatonin secretion increases along with the number of hours of darkness. The normal cycles may also be interrupted by changing one's daily habits, for example changing feeding time, following a gradual force-to-sleep or sleep depriving.
In addition to these major influences there are a variety of other environmental factors that may have an impact on biological rhythms. One of them is caffeine. A series of experiments on caffeine revealed differences in the effects of the drug depending on time of day. In the morning caffeine has been shown to hinder low impulsiveness, while the opposite is true in the evening [47]. This finding suggests that low impulsiveness and high impulsiveness differ in the phase of their diurnal rhythms, resulting in a difference in the effects of caffeine.
By establishing an understanding of various environmental factors that influence biological rhythms it is possible to draw connections between the significant time shifts and changes in nature and mood disorders.
Moreover, the influence of those factors may be quantified by developing a hybrid measurement system incorporating measures of brain activity, heart rate, and respiration as well as changes in the level of adrenalin in the blood over time.
2.6 Summary and Conclusions
Physical, biological, and mental biomarkers of the human body can well describe its state. Body movement, heart rate variability, and the brain responses to various internal and external stimuli can reveal the symptoms and causes of many abnormalities in the state of human body. To differentiate these abnormalities and disease indicators, however, a variety of tests and measurements by means of suitable sensors need to be undertaken. These indicators can be quantified using data-processing and intelligent systems for better and quicker diagnosis of the abnormalities in humans. Although sensors and sensory networks have facilitated recording and quantification of many of the states indicating variables, there is still a long way to go to cover all factors involved in the full recognition of human body states.
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