**Author details**

metabolic disorders such as obesity and metabolic syndrome are seen together [27, 29]. It has been reported that disrupted biological clocks lead to intolerance of glucose, insulin, and lipid metabolism [27–29]. Because of the glucose homeostasis is dependent on daily light–dark cycle, in case of the desynchrony glucose regulation impaired and this situation leads to metabolic syndrome even diabetes mellitus [12, 23, 29]. In addition, it has been reported in many studies that depressive and affective disorders were accompanied by disrupted sleep–wake patterns [10, 31]. In case of circadian dysrhythmia, the normal morphology of biological rhythms are changes in one or more of the aspects of the normal cycle. This could be a change in degree of fluctuation (mostly decreased amplitude), a phase shift (mostly phase delay), or disintegration of the cycle in a chaotic pattern. It has been shown that the severity of illness was correlated with degree of circadian disruption [7]. Scientist gradually understands that variety of the diseases are rhythmic disorders [10, 28–31]. The sleep disturbance is the first and overt symptoms of circadian dysrhythmia. The core temperature is one of the stable rhythms. It decreases consistently around 5 am. However, in chronic diseases fluctuates. It has been shown a correlation between severity of illness and degree of circadian disruption [7]. Cardiovascular system is a good example of organizing according to the oscillation of biological clocks. Most cardiovascular functions change with the circadian cycle. The longer-term effects of disrupted circadian rhythms are increased risk of obesity, metabolic problems, depression, mental disorders, cardiac and neurological events and even cancer. In recent years, there is a growing awareness about the negative effects of the technology to biological rhythm. Chronic exposure to artificial light causes numerous health problems from simply sleep disorders to many cancers [32]. It is known that sleep–wake disruption common in industrialized society depends on lifestyle. The main issue is how human being could keep their biological clocks in a regular system. As understood clearly in the biological cycle of human body, the daytime is optimum for food intake because of the biological rhythms peaks in the morning and afternoon [12]. Froy et al. showed that well-being could be achieved by resetting the circadian clock. They reported that timed feeding also arranges circadian rhythms [30]. In a similar way, it is obvious clearly the sleep and activation time. It is well known that there is a strong correlation between disrupted circadian rhythms and many health problems person. Based on this knowledge, scientists (consider the contrary) begun to search whether healing be guided by circadian rhythms as well. It has been shown that chronobiological interventions improved the clinical outcomes through amelioration of delirium and sleep disturbance [7, 30]. Understanding what makes biological clocks tick may provide treatments for many health problems from sleep disorders, obesity, mental health disorders, to jet lag. It can also improve ways for individuals to adjust to nighttime shift work. Learning more about the genes, which is responsible for circadian

10 Circadian Rhythm - Cellular and Molecular Mechanisms

rhythms, will also help us understand biological systems and the human body.

• In a molecular level, there are thousands of biological clocks in the human body.

**2. Summary points**

• Human being has a diurnal activity pattern.

Hülya Çakmur

Address all correspondence to: hulyacakmur@gmail.com

School of Medicine, Department of Family Medicine, Kafkas University, Kars, Turkey
