Around a quarter of you donates not only their daily heart rate and steps but also their sleep data, particularly how long and when you sleep. Since sleep provides a wealth of information on health and well-being, we are now taking a closer look at these data - enlisting also the expertise of sleep researchers.
Our analysis of the sleep data has two main aims: First, we would like to understand how sleep is changed under the pandemic - because of lockdowns, home office and psychological burden or distress. We hope that this will provide indications about general well-being and health during the pandemic as well as side effects of pandemic measures. Second, we want to further improve our fever monitor based on the sleep data. Someone who is acutely sick usually sleeps more than before. We seek to make use of this effect in our real-time monitoring of COVID-19 cases in Germany.
In this blog post, we would like to introduce the sleep data in general.
How do we sleep?
Sleep has multiple dimensions. Among those most easily measurable are sleep duration and sleep timing. Sleep duration refers to how long one sleeps; sleep timing refers to when one sleeps i.e. the midpoint between first falling asleep and the final wake-up. Both sleep duration and sleep timing change with age and differ between women and men. This is also the case in the data from the donation-data:
Sleep duration: You can see that young donors sleep on average the most and with increasing age, sleep gradually shortens. Across most ages, women sleep slightly longer than men. The group showing the shortest sleep duration are 60-year-old men with 6h 59 min. In contrast, the group of 20-year-old women sleeps on average half an hour longer at 7h 30min.
Who is the most pronounced night owl, can fall asleep only late and has trouble getting up in the morning? No surprises here: the teenagers and young adults show the latest sleep timing also in the donation-data (shown is the sleep timing on weekends). With increasing age, sleep becomes earlier and earlier, so owls slowly transform into larks. Generally, men tend to be more owlish than women i.e. sleep later. The earliest midsleep time is exhibited by 70-year-old men at 03:40. In contrast, the latest midsleep time, over one hour later, can be seen in 20-year-old men at 04:50. If one assumes a fixed sleep duration of 8 hours, a young man would sleep on average from 00:50AM to 08:50AM on his weekends, whereas a man well advanced in years would sleep rather from 11:40PM to 07:40AM.
Sleep timing is regulated by a body clock - set by light
When and how we sleep is not our free decision but subject to strong biological regulation. Our body clock, also called circadian clock, provides our body with an internal time-of-day: during the biological night, our body is programmed for sleep, during the biological day, it is programmed for wakefulness. The circadian clock is mainly set by light - the best and strongest light for synchronisation is sunlight.
The donation-data show quite nicely that our sleep is indeed influenced by sunlight. There is a clear pattern from East to West in sleep timing. In the East, where the sun rises and sets earlier, people sleep earlier on their free days (weekends and holidays) than in the West, where the sun rises and sets later (around 4 min per longitude).
The cities stand out from this pattern: Here, sleep timing is significantly later than in the surrounding, less populated areas. Several studies indicate that this is at least partly due to lower daylight exposure in densely populated areas. A weaker light signal leads to a delay in the circadian clock and thus also in sleep timing. If opportunities for evening leisure activities also play a role is not clear, however, during pandemic times, there are much less such opportunities…
The town Sömmerada in Thuringia shows the earliest sleep timing with an average of 03:12. The people in the city of Mainz in North Rhine-Westphalia sleep almost a full hour later with an average of 04:11. For this comparison, however, one needs to take into account also the differing demographics between the regions.
Living and sleeping in 2 time zones: jetlag at home without any travel
Much stronger than the East-West pattern, however, is the workday-weekend pattern in sleep timing. This can be seen also in the plot of the daily sleep timing in the donation-data: on weekends, donors slept not only longer but also much later than during the week.
The strong weekly pattern shows also interesting deviations: on public holidays in May and June, and most strongly over Christmas and New Year’s. As expected, the latest mid-sleep time was on January 1st at 05:29 after New Year’s celebrations. The earliest sleep timing was found at 02:47 on the day after the change from Daylight Saving Time to standard time at the end of October. This means a difference of 3 hours in average sleep timing across the year! What were the effects of the recent change into DST? We are most curious and will look into the DST effects more closely in a future blog post.
Since the weekly differences in sleep timing between workdays and free days remind of travel across time zones, this phenomenon is also called “social jetlag”. It seems that we literally travel between two time zones each week, one work time zone and one leisure time zone. According to our preliminary analyses, the north-westerly regions of Germany seem to be a bit more affected by social jetlag than the south-easterly regions. In southeasterly Upper Bavaria (Bavaria), donors show on average 45 min of social jetlag, in northwesterly Weser-Ems (Lower Saxony), they have 56 min of social jetlag. Also here, demographic differences between the regions need to be considered when interpreting the data.
How does COVID-19 and pandemic measures influence sleep and these patterns? We will take a closer look in one of our following blog posts.