Effects Of External Environment On Circadian Rhythm And Sleep Timing Of Different Chronotypes

Circadian rhythm is an endogenous rhythmic behavior of organisms for adapting to daily environmental variations which widely exists in all kinds of life activities in nature.Circadian rhythm is affected by external environment,and especially can be entrained by light-dark cycle.The circadian rhythm regulates almost all physiological activities and behaviors of organisms,such as sleep-wake,body temperature,blood pressure,and so on.The disruption of circadian rhythm leads to dysfunction and even disease.Relevant research helps us better understand the life phenomenon,reveal the internal mechanism of biological organism adapting to the environment,and improve the diagnosis and treatment level of diseases related to circadian rhythm disorder.The timing of the human sleep-cycle is regulated by both endogenous circadian clock and homeostatic processes.Light is an important factor affecting sleep time and circadian rhythm.Before industrialized society,the light-dark cycle is determined by sunrise and sunset,and people had enough sleep timing.Nowadays,we often sleep late and it is hard to wake up on time.After entering the industrialized society,humans can extend the lighting time by turning on artificial lights,and the waking time is usually triggered by an alarm clock to meet the social schedule.This modern lifestyle is believed to be related to the time mismatch between sleep and circadian rhythm(social jet lag)and insufficient sleep,which will lead to bad psychological and physical health results.With the help of mathematical model and computer simulation,this paper studies two aspects: entrainment ability and mechanisms of circadian rhythm and light-dark cycle,and comparison of the sleep timing of three chronotypes affected by modern lifestyle.Entrainment ability and mechanisms of circadian rhythm.Light-dark cycle is the key factor that regulates the circadian rhythm.We studied the entrainment of the circadian rhythm system.The results show that only when the external periodic light intensity reaches a certain threshold can the circadian rhythm synchronize with it and has a period of 24 h.The inherent period of the circadian oscillator and its ability to receive light affect the synchronization ability of the system.The smaller the difference between its inherent cycle and 24 hours,or the stronger the ability to perceive light,the easier it is to synchronize with the light-dark cycle to 24 hours.By comparing the phase response curves under different parameters,we find that the phase shift generated by the same light pulse is also positively correlated with the system’s ability to perceive light.When the external periodic light suddenly changes,the reentrainment ability of the system is also positively correlated with the system’s ability to perceive the light.Comparison of the sleep timing of three chronotypes.According to preferred sleep-wake schedule,there are three different chronotypes.Morning-type individuals go to bed early in the evening and get up early in the morning.Instead,evening-type individuals go to bed late and get up late in the morning.Normal-type individuals are neither evening-type nor morning-type.In this paper,we apply a mathematical model including circadian rhythm,sleep homeostasis and environmental light to get a quantitative understanding of the effects of modern light consumption and social constrains on sleep timing of people with different chronotypes.The results show that the prolonged day light and evening light exposure both delay preferred sleep timing with the sleep duration almost unchanged.People of evening-type or with longer intrinsic periods are most expected to be vulnerable to evening light.Increasing light exposure can offset the effect of evening light to some extent,but it is most difficult for evening-type people.Social constrains cause the largest social jet-lag in people of evening-type,which increases with evening light intensity or intrinsic periods.Morning-type people’s sleep symptoms worsens,while that of evening-type people improves with age.This study provides a theoretical reference for preventing and treating sleep disorder and social jet-lag for individuals with different chronotypes.