Social Jetlag
Social jet lag (SJL) is a measure of the discrepancy between the body's internal clock and the social time [WDMR06]. Everyone who needs an alarm clock to get up suffers from that [Diem00]. In this case, the social requirements for when to sleep do not correspond to the sleep times that the body would choose on basis of its sense of tiredness and therefore of the internal clock.
Figure 1 shows an example of a sleep diary of a person with strong social jetlag. This person's sleep time on work days (dark gray) differs significantly from that on weekends (light gray). On work days, sleep is shortened at both ends: At night, the person's internal clock does not allow them to fall asleep at the right time [RPZW19]. In the morning, the alarm clock violently stops the sleep before completion [RPZW19]. The result: chronic sleep deprivation, which can only be partially compensated at the weekend [FPWW13, RoWK19].
​
At this point it should be noted that sleeping in at the weekend is not a reinforcement of the problem, but a relief. In other words, it is healthy. People with too little sleep during the week have a higher death rate if they do not get compensatory sleep at the weekend than their fellow sufferers, who can at least partially catch up on sleep at the weekend [ÅGGZ19].
Figure 1: Sleep diary of a person with high social jetlag. Source: [RPZW19], edited.
The social jet lag is the calculated difference between the average midpoint of sleep (midpoint between falling asleep and waking up) on workdays (green in Fig. 1) and on weekdays (red in Fig. 1) [RPZW19, WDMR06].
Figure 2 shows a statistic on the social jet lag of the population. Only approximately 20% of the population suffer of less than half an hour of social jet lag and get on working days more than 7 hours of sleep [RoWK19].
​
Social jetlag is comparable to travel jetlag, which you experience when traveling long distances to the east or west. The difference is that travel jetlag disappears within some days after arrival at the destination, because the body adapts to the new light conditions (sun time), while social jetlag is chronic. [RPZW19]
​
Social jet lag is associated with health risks (see Health Consequences of Social Jetlag) [Roen19].
Figure 2: Sociale Jetlag of the population. Source: [RPZW19], edited.
Causes of Social Jetlag
The cause of social jet lag is a combination of inappropriate social times and internal clocks being delayed by light pollution.
The body's internal clock is set almost exclusively by light and not by social times, even in industrialized nations. The solar time (time of sunrise and sunset) is accordingly the strongest factor influencing the internal clock. [RoKM07]
Unfortunately, social times barely align themselves with the time of the sun. Furthermore, there are also individual differences in how the individual's internal clock sets in certain lighting conditions (see Chronotypes).
Cause 1: Time Zones
​
When defining time zones, social time is partially decoupled from solar time. Within a time zone, all people have to get up at the same time. However, the sun rises and sets later at the western border of the time zone than at the eastern border. If the standard time zones, as decided in 1884, are applied, this time difference is one hour. This means that people in the west of the time zone have to get up an hour earlier than their fellow citizens in the east. As a result, social jet lag increases from east to west within a time zone [Roen19].
Cause 2: Choice of geographically incorrect Time Zones, Daylight Saving Time
The problem of decoupling social time from solar time is exacerbated by the choice of geographically incorrect time zones. Countries are often in time zones that are much further east than the country’s position. Spain, France, and the Benelux countries are examples of this. They are located geographically in Western Europe, but were assigned to the Central European Time zone in the Second World War. Therefore, in western Spain sunrise and sunset occur two full hours later than on the eastern border of the time zone (where Warsaw is approximately located). This increases social jet lag enormously. An instinctive reaction of many affected cultures is a shift in daily routines. It is not for nothing that Spaniards are known to eat dinner very late.
Daylight saving time corresponds to choosing a time zone that is too far east. Countries in Central Europe (e.g. Germany) live during the summer time with Eastern European Time. Consequently, the social time is in that case ahead of solar time. Therefore, social time is too early for our internal clock.
​
Cause 3: Individual Differences
​
As with any physical characteristic, there are also genetic differences in how the individual's internal clock sets in certain lighting conditions. Early chronotypes (larks) get tired early and wake up early. Late chronotypes (owls) can only fall asleep late and expectedly also wake up late. However, our social times are made almost exclusively for very early chronotypes (night rest 10 p.m. - 6 a.m., usual work or school start 8 a.m. or even earlier). Medium and especially late chronotypes therefore suffer from chronic social jetlag [WDMR06].
​
Cause 4: Light Pollution
​
The light-dark change is the main zeitgeber (time signal) for our internal clock. With less time spent outdoors, often due to the professional reasons, and artificial light, this natural zeitgeber of our internal clock is weakened [Roen19]. As a result, most people's inner clocks are later today than under natural light conditions [SCMM17]. Consequently, there are more late chronotypes (owls) and therefore more people who suffer from social jetlag. Only extreme larks grow earlier under this weakened zeitgeber. [Roen19]
​
In addition, the body needs in particular light in the morning to set the internal clock earlier [RoDM03]. However, for a large part of the year, many people have to be at work or school before daybreak and therefore have no way of getting this light.
​
The possibilities to avoid weakened light-dark changes in our modern society are very limited. Of course, you can try to increase the time spent outdoors (especially in the morning) and reduce the use of artificial light in the evening (see light hygiene). But do we want to abolish all professions that have to be carried out inside buildings? Do we want to encourage people to have dinner with only candle light in during winter months? Should we completely ban moderate TV viewing in the evening? Probably not. Then a logical consequence is to adapt social times to the physical needs of people in our modern society.
Health consequences of social jetlag
A large number of studies show the negative effects of social jetlag on physical and psychological health. Since late Chronotypes suffer more from social jetlag, they are most affected [Part15, WDMR06].
​
Social Jetlag
-
increases the risk for
-
affects our immunity negatively leading to:
-
impaired immune responses and higher susceptibility to infection and disease [BCSN17, HABC20, PaPD18, PhSK15].
-
an exacerbation of allergies, like asthma, allergic rhinitis, and atopic eczema, and autoimmune responses, like rheumatoid arthritis, and inflammatory bowel disease [BCSN17, CKSD18, HABC20, PaPD18, PhSK15].
-
-
Increases the insulin requirements of adolescent diabetes type I patients [SBBK18].
-
Increases the consumption of cigarettes, caffeine and alcohol [WDMR06, WiPR10].
-
Increases the tendency for unhealthy eating habits [APWV18].
-
Affects cognitive and academic performance [DíEs15, HEGR14].
-
Increases the physical and verbal aggression potential in young adults [RaVo13].
-
Reduces the life expectancy [Bori11a].