During the first three parts, we covered the different reasons that give rise to different chronotypes. In this last part of the lecture we will discuss how modern lifestyles impact not only entrainment, but also our entire sleep-activity behaviour. I had mentioned earlier that sleep timing and sleep duration are different independent traits. So far, we have focused predominantly on sleep timing, but we will now look at sleep duration. Similar to mid-sleep times that we use to assess chronotype, sleep duration has also a relatively wide distribution in the population. The distribution you see here represents sleep duration averaged over the course of a week. The majority of the people represented in the MCTQ database sleep somewhere between 6.5 and 8.5 hours. But there are a significant number of people who state that they sleep -on average- four hours or even shorter. At the other end of the distribution are people who apparently need ten hours or even longer. The next graph compares the distribution of weekly averages with the sleep durations on workdays. You see that this distribution is slightly displaced towards shorter durations and that the number of people who indicate to sleep only four hours or shorter increases significantly. As to be expected, the distribution of free-day sleep durations moves in the other direction. Mean and median become later, and the width of the distribution becomes much wider. More than 25% of the population, now indicates to sleep 9 hours or longer, and as much as 1%, almost sleeps half of their free days. Not that this is not time in bed, but actual subjective sleep duration from falling asleep to waking up. We have already looked at the marked difference between workday sleep and that on free days. But I would like to take a closer look at this phenomenon with the help of sleep logs that subjects kept for approximately a month. This sleep log was kept by an extreme late type, who can choose his own work hours. He regularly falls asleep around 4 AM and wakes up some time after 11 AM. The red bars represent sleep episodes on workdays and the green bars are those on work-free days. Note that both sleep onsets and sleep ends scatter, as to be expected in a biological trait, and that there is no marked difference between sleep on workdays and that on free days. The next sleep log was kept by an extreme early type, who regularly falls asleep between eight and 9 PM And tends to wake up at around 5 AM. The comparison between the first and the second sleep log are so drastic that the two could almost share a bed without overlapping. Again sleep onsets and sleep ends show up on a biological scatter, but in this case one can see that this early type sleeps too late on free days. This is probably due to the much later population that makes extreme early types go to bed much later than their circadian clocks tell them to do so. But despite of having gone to bed far too late, the circadian clock of early chronotypes usually wakes them up at their usual time. While these two people represent exceptions, the majority of us would produce sleep logs similar to the one shown in the next graph. Please look at this person's wake-up times on work days. They show absolutely no biological scatter and are so regular that one has to assume an alarm clock. The most apparent quality of this sleep log is, however, the enormous displacement of sleep timing between work- and free days. It is as if this person flew many times zones to the west each Friday evening and flew back on Monday morning. Since this virtual travelling is similar to a jetlag situation without the people ever leaving their location, we have called this phenomenon "social jetlag". A simple way to quantify "Social Jetlag", is to take the difference in sleep midpoints on workdays and on free days. The distribution of "Social Jetlag" in the MCTQ database indicates that only 13% of the population do NOT suffer from this modern syndrome. You have, earlier in this lecture, seen the age-dependency of chronotype with the late teens and early tweens being the latest chronotypes. It is therefore not surprising that the peak of social jetlag is reached at similar ages. While the circadian clock is latest in these age groups, many of them have to go to school even earlier than most of the working population. In the next graph we return to sleep duration. Again, the results that this graph show are not really surprising. Children need more sleep than older people. The surprise comes when we separate sleep duration for each age group on workdays and on freedays. Especially teenagers are forced to sleep far too short on workdays and therefore have to catch up on their sleep loss by sleeping half of their free days. This is probably a relatively recent development. I have shown you the consequences of industrialisation that drastically reduced zeitgebeber strength, so that our clocks have become much later, while our social schedules have remained relatively early. The consequences that we cannot fall asleep as early as we should to fulfil our sleep requirements, but still have to get up early. More than 80% of the population of the people in the MCTQ database use an alarm clock on workdays, and the use of an alarm clock simply means that our biological sleep time is not over yet. It is remarkable how much we mistreat our circadian and our sleep needs by simply ignoring the consequences of modern technology and of modern decisions. This slide shows the change in social jetlag across the year. Since our clocks are later in winter than in summer, we suffer from more social jetlag in winter. You must have seen the large peak around the spring equinox and asked yourself what this peak represents? This peak is a reaction to the spring transition to daylight savings time. Although, we speak of a "time change", "time" is actually not changed, at least sun-time isn't, which is the original basis for time. All we do during DST is to go to work an hour earlier, which adds to our social jetlag. We will talk about the health consequences of these modern challenges on the circadian clock and sleep in the next lecture. [SOUND]
