On Social Jetlag
We need regular sleep
Numerous studies have shown the importance of regular sleep. Not only we need sufficient quantity of sleep, we should also go to bed at and wake up at a regular hour. Various metabolic processes in our body follow cyclic daily patterns (circadian rhythm), and sleep has its own place in it. We all experienced the discomfort caused by traveling via several timezones when our circadian rhythm got out of sync and it took several days to adjust. And it is not only about subjective discomfort, irregular sleep can have serious health impacts, such as increased risk of obesity, diabetes, and cardiovascular diseases.
Till Roenneberg, a professor of chronobiology at Ludwig-Maximilian University in Munich, makes an argument that people are born with a chronotype. Everyone is somewhere on the spectrum from owl to lark, and this doesn’t change by any external pressures. Not adhering to one’s natural chronotype has severe consequences to the quality of life, especially concerning fatigue and energy levels.
In his excellent book Internal Time, Roenneberg pioneered a concept of social jetlag. It describes a notion that when people have a completely free day, they will turn to their natural chronotype. But when they work on a fixed schedule, they have to fight this natural tendency on workdays. This constant shifting of sleep pattern between workdays and weekdays creates a certain tension that can be best compared to a periodic shifting of timezones – and jetlag-like results. See more on this topic in our introductory article.
The sleep study
In our anonymized sleep database, we have more than 10 million records capturing our users’ sleep habits. What are their sleep patterns? How many of them are socially-jetlagged? In order to get some insight on this topic, we explored the data of about 20000 users from whom we have at least 200 records. Let’s see a few typical cases.
Peter, the perfect sleeper
The chart below depicts about 6 months of sleep history of a particular user – let’s call him Peter. Each blue bar represents one night’s sleep, spanning between the time when Peter started sleep tracking and the time when he stopped it. This is an example of nearly ideal sleep habits, very orderly and regular. He goes to bed around 1 am and wakes up at 8 am. There is very little variance, very few nights when he departs from his habits and goes to sleep a couple of hours later. Only a tiny minority of our users – less than 10% – has such a nice and regular record.
Olga, the jetlagged owl
Let’s see another example, from a completely different rank. It looks like we have a typical night owl, struggling to balance her internal rhythm with her work schedule. Most days, she wakes up exactly at 6 am, apparently with alarm. But she goes to bed at 1 am in average, so she gets only 5 hours of sleep at a workday. Although this is probably not enough for her, she does not go to bed earlier, she simply would not be able to fall asleep due to her innate late chronotype. Then, after 5 such days, there are always 2 happy weekend days, when she can act according to her natural disposition. She goes to sleep around 3 am and wakes up at 11 – after 7 hours of sleep. We can also see two longer periods – probably holidays – when she completely switched to her natural mode.
This is a typical example of social jetlag. Due to her duties in the society, Olga shifts her sleep time several hours forth and back every few days, as if she flew between New York and Los Angeles every weekend. It is likely to impact her heath and overall well-being.
What can she do about this systematic mismatch? She can not reprogram her inborn chronotype. She can only adjust her habits in order to reduce the lag. Finding a job that allows her to wake up later would certainly help. There are also techniques that may help some people to fall asleep earlier, such as physical exercise or exposure to red light.
Olga is rather a pronounced and extreme case, but we found out that about 30% of people in our database have two distinct sleep patterns (corresponding to free/working days) and they are constantly switching between them. Most of them are owls (they go to bed and wake up later on free days), as the natural rhythm of larks tends to be closer to a typical workday schedule.
The free/working days division do not necessarily correspond to weekdays, weekends, and public holidays. Some people have non-standard work schedules, everyone takes a holiday sometimes, etc. We detect these patterns dynamically from the data, based on typical habits of particular users.
Iris, the irregular sleeper
However, how about the remaining majority (about 60% of the users) who neither sleep regularly, nor they have two distinct sleep modes – a systematic social jetlag? The most typical sleep record looks something like this.
Iris usually goes to bed around 2 am and wakes up around 8 am, but there are many days where she diverges from these times by several hours. There is no apparent regularity detectable, no weekly patterns, it just randomly fluctuates in both directions, based on her particular duties or mood. There are some blank spaces in the chart when she did not track her sleep, so we might be also missing some important information.
The original Roenneberg’s research was based on self-assessment questionnaires, where 25000 volunteers stated when they go to bed and wake up on working days and free days.
Our data suggest that most people actually do not have any such regular patterns, so the original notion of social jetlag does not make much sense for them. Most people are more like Iris, rather than like Olga.
However, most users do not sleep as regularly as Peter the perfect sleeper. Their record exhibits large random fluctuations in their sleep duration, beginning, and end. Therefore we introduced a more general measure – sleep irregularity – in our software. Technically, it is a formula combining the standard deviation of sleep length and wake-up hour over a period of time. Generally speaking, it’s desirable to reduce the irregularity and strive to get to a habit of more regular sleep.
Discover your patterns with Sleep As Android
In our app Sleep As Android, we introduced a number of features that help our users to discover their sleep patterns, and to track their progress in taming their sleep irregularity.
We assess the user’s chronotype from the average sleep hours, and we plot a long-term chart of her sleep irregularity and social jetlag. See more information on the topic here.
Now, how would these statistics look like for our three heroes, Peter, Olga, and Iris?
Peter’s sleep irregularity would be about 30 minutes and his social jetlag (the difference of mid-sleep on free and working days) would be close to zero because he does not exhibit any sleep habits variation between free and working days.
As for Olga, both her social jetlag and her irregularity would be about 2 hours, as all here sleep variance is caused by her periodic sleep mode switching.
The irregularity of Iris would be also about 2 hours, indicating rather bad sleep habits, but her social jetlag would be close to zero. There is no detectable regularity in her habits, so our algorithm, as a fallback, calculates the difference of mid-sleep between weekdays and weekends, and the random variations mostly cancel out.
Sleep irregularity is a general measure of orderly sleep habits, while social jetlag makes sense only for people who exhibit a certain kind of periodic sleep pattern switching.