The nerve center responsible for controlling our circadian rhythm is located in 
a cluster of cells known as the suprachiasmatic nucleus in the hypothalamus. 
Various physiological functions such as sleep, arousal, blood pressure, body 
temperature, and hormone secretion are regulated in a 24-hour rhythm by this 
circuit. Somatic cells of other organs have a peripheral clock gene and by 
synchronizing the rhythm of the central and peripheral clocks, it is possible to 
live a healthy life. Due to aging and degenerative disease, circadian rhythm 
gradually collapses. Factors that can contribute to this include reduced 
expression of the time gene associated with photo stimulation, a reduction in 
neurotransmitter levels, and reduced melatonin production. Biological clocks 
play an important role in our emotions, cognitive function, and behavior. Sleep 
disorders and metabolic disease related to the circadian rhythm affect metabolic 
and endocrine activities via the autonomic nervous system and the intestinal 
bacterial flora. Shift work disorder is associated with insomnia and excessive 
drowsiness as individuals often work during their sleeping hours. Now time 
management is placed at the center of our society, and it is important to 
evaluate the medical risk of engaging in shift work. In frontotemporal dementia 
(FTD), the stereotypical behaviors may be associated with time. In some 
patients, multiple timed behaviors occupy a considerable part of the patient's 
daily life. Stereotypical behaviors in FTD are often considered in contrast to 
obsessive-compulsive disease (OCD). Studies of OCD have found a close 
correlation between clinical symptoms, cognitive function, and brain function.