Like all other living things, humans have a circadian clock. The rhythmicity of the clock is resynchronized daily with the 24-hour rhythmicity of the day by means of timers such as light and temperature. Problems with resynchronization can cause significant discomfort, such as depression.
What is resynchronization?
Problems with resynchronization arise, for example, after long-haul flights. The internal clock no longer matches the prevailing day-night rhythm after a trip to another time zone. Circadian rhythm is also known as the internal clock. It adjusts the human organism to daily recurring phenomena. The circadian clock controls not only the heart rate, but also the sleep-wake rhythm, reproduction, blood pressure or body temperature. The organism thus moves in time largely independently of external influences and performs periodic activities with a relatively constant rhythmicity. A genetic basis controls the period length. However, for the rhythm of the internal clock to actually match the 24-hour cycle of the day, constant resynchronization of the circadian clock is required. This resynchronization is especially necessary against the background of changing day lengths during the course of the year. Mainly the photoreceptors in the outer granular layer of the retina are concerned with resynchronization. Light and its changes are thus used as circadian zeitgebers for resynchronization of the internal clock. Since the period length of the internal clock is not exactly 24 hours, a lack of readjustment or resynchronization causes the organism to fall out of rhythm. In addition to humans, animals and plants also adjust their rhythms to the day-night cycle via the circadian clock and its automatic readjustment.
Function and task
In plants, the photosynthetic apparatus is activated before sunrise, preparing the organism for the onset of photosynthetic activities that can be performed exclusively in daylight. Some plants open or close flowers at specific times of day or produce nectar at a specific time of day. The existence of a free-running circadian rhythm at constant conditions leads scientists today to assume the existence of a rhythm-generating internal unit. According to current findings, this control unit is located in the central nervous system. In mammals, the control unit of the circadian clock is probably located in the nucleus suprachiasmaticus of the hypothalamus. From here, all other circadian pacemakers in the periphery are coordinated. The molecular clock functions according to a transcription-translation feedback loop. Protein translation inhibits the transcription of the involved genes for the respective protein. The key proteins involved, in addition to CLOCK, BMAL1 and PER, are thought to be CRY and NPAS2. The feedback sequence of the molecular coupling mechanism takes about 24 hours. Indirect neuronal and hormonal signals, along with temperature changes and light, synchronize these temporal sequences. Because the external cause of circadian rhythmicity is the intrinsic rotation of the planet, the most relevant external rhythm generator is the variable illumination intensity of the atmosphere. The visual system detects this pacemaker. This makes light probably the most relevant and universal zeitgeber for resynchronization of the internal clock. When it is evening or night according to the internal clock, but the retina still detects light, the internal clock is resynchronized. In this way the organism can adapt to the changes of seasons. The resynchronization of the internal clock is necessary for numerous body processes. Accordingly, failure to resynchronize can have serious consequences.
Diseases and ailments
People are particularly susceptible to problems with circadian clock resynchronization because of their lifestyle. In particular, people’s modern lives easily throw the circadian clock out of balance, which can have a negative impact on how they feel and their health. Problems with resynchronization arise, for example, after long-haul flights. After a trip to another time zone, the inner clock no longer matches the prevailing day-night rhythm. Resynchronization must take place at short notice. Jet lag is a consequence of these interrelationships. Shift work poses a similar problem. Shift workers live in opposition to their inner rhythm.People also spend less and less time in daylight. Especially in winter, the light intensity indoors is hardly higher than 500 lux. At night, modern humans are permanently exposed to artificial light stimuli. The daily resynchronization of the inner clock is often confused because of these interrelationships. In addition to sleep and eating disorders, resynchronization problems promote lack of energy and even depression. Metabolic stress can also be a consequence of resynchronization problems. As secondary diseases, diabetes mellitus and obesity are thus favored. A particularly well-known primary disease associated with resynchronization is circadian sleep-wake rhythm disturbance. Sufferers of these sleep disorders are unable to fall asleep when sleep is desired or required. When wakefulness is required or expected, they are sleepy and can barely keep their eyes open. This phenomenon is most likely to occur as part of the shift worker syndrome or affect frequent travelers with regular jet lag experiences. Two different types of sleep disorder are distinguished. While one is characterized by delayed sleep phases, the other is characterized by pre-delayed sleep phases. Since blind people have a much harder time resynchronizing compared to sighted people, circadian sleep disorders affect them significantly. If left untreated, sleep disturbances can trigger a number of psychological and physical sequelae in the long run.
