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I always emphasize the fact that we very often focus on one or two things that we think will help us feel better and be fitter, but we forget about much more important things. Diet and movement are important factors when it comes to feeling energetic, losing weight and looking good. And yet, diet and movement are just the incoming information that the body uses in a certain way. How the body interprets the information depends on its condition and on how all systems in it are functioning. You know I like discussing human behavior and the human body through the prism of circadian rhythm or, in other words, the inbuilt, biological clock that everything in nature obeys.

This week we are going to experience an event that occurs every year – the time shift when we have to adjust our clocks forward one hour. You all probably know how this shift feels and that you feel somehow more tired, and while you can’t wait for the time shift in October, when you get to sleep one hour more, you are loath to adjust your clocks forward, when you have to get up an hour earlier.

The thing is that when we advance our clocks by one hour, we make no change to the environmental factors, for example sunshine duration or rather light frequency. This may sound confusing, but before I lose you as readers: you all know that light frequency is different at different times of day. This very light frequency is one of the main environmental factors that set our biological clock and synchronize our bodily functions with the environment we find ourselves in.

Our circadian rhythm – our biological clock – follows the sun and the changes at the geographic latitude where we are. Actually, it changes at four-minute intervals, which is the time the sun takes to cross one latitude line.

If you didn’t study German or are not interested in chronobiology, you have probably never heard about the word “Zeitgeber” (time setter) which researchers use to describe various signals in the environment that can affect our schedule. A very important fact is that light is one of the main Zeitgebers in setting the biological clock. What is characteristic of sunrise and sunset is that, apart from very beautiful views, light at these times of day is very important for the body. Even when we adjust our wristwatch, our biological clock stays connected to sunrise and sunset.

1.Time shift and hormonal shifts

Advancing time by one hour leads to daylight till later in the evening, but less light in the morning. For the body this is unnatural and even though one hour doesn’t sound as much of a difference, it is huge in terms of bodily processes.

You probably know that levels of cortisol (i.e. the so-called stress hormone) should peak in the morning, because this is one of the ways we wake up. But in fact one of the main factors that regulate cortisol levels is light. It is sunrise and light that lead to an increase in cortisol secretion and wake us up.

When it is dark, and we have to re-adjust to this change overnight, the hormones in our body can’t do it. This leads to a discrepancy between what happens in the body and what the environment expects from it.

The other factor is that after the time shift it is lighter outside in the evenings. In order for the hormone melatonin (the one that makes us sleepy) to be released, it has to be dark. This hormone starts to be released after sunset, and when sunset comes later in the day, we don’t get sleepy early in the evening, so we are a lot more active till late at night. Our bodies stay synchronized, but not with the time on our watch, but with time in nature – sunrise and sunset.

2.Time shift as a change in latitud

Adjusting time forward one hour can theoretically be compared to a change in latitude. Disruptions as a result of DST (daylight saving time) increase latitude. The 1 hr DST advance in spring corresponds to traveling 15° westward and the reduction of amplitude corresponds to traveling 17° latitude southward. This means that our geographic latitude changes, but the climate and nature don’t.

In the graph you can see how the sleep cycle changes during the DST period and during the Standard European Time period. The DST period is marked with the white triangles, while Standard European Time is marked with the black ones. The gray color corresponds to dawn and you can see that during the Standard European Time period sleep follows the dawn, while during the DST period mid-sleep is moved to a much earlier time – approximately 3:30. And in graph B you can see how sleep duration decreases during the DST period.

mid sleep


3.The immune system and time shift

Also, you have probably noticed that you are more susceptible to catching colds in the spring. Of course, this cannot be claimed with a 100% certainty, but DST affects the immune system as well – it, too, is directly connected to light and sleep. You may not know this, but the night, and especially the time we are asleep, is one of the main times when the immune system performs its functions. Actually, the immune system is one of the things that make us sleep. You can also think about the times you are ill and feel sleepy and that sleep is what helps you recover faster – that is when the immune system does its job. When sleep is disrupted, this affects all systems in the body and that is why not getting enough sleep can have negative consequences for the body.

And last, but not least, change in one group of hormones leads to a cascade of changes in the levels of other hormones. If you read this post, you already know how lack of sleep affects appetite.

What can we do?

In our chronically undersleeping society, advancing the clock by one more hour definitely has a negative impact on our body and the way it functions. Of course, we can’t make the decision not to shift the time. But we can stop and think about how this affects our body and do the bare minimum to help it deal more easily with the shift.

The most important thing is to spend enough time in daylight – at sunrise and in the afternoon hours before sunset, if possible. Even if you don’t have time to take walks, you can always go out on the balcony or in front of your office and use your 10 or 15-minute break to be outdoors and not indoors. Don’t forget that light is the best way to set the biological clock.

Also, a brief spell of physical activity at midday or in the early afternoon hours has a positive impact on setting the biological clock.

It matters when you go to bed, too, so make sure you go to bed a little earlier at least on this weekend, so you can get enough sleep, even though you have to get up earlier.

And on Monday keep in mind that your appetite may increase due to this change and instead of restricting your food intake, read here how you can select the right foods to feel full and not let this interfere with your progress.

If you liked this blog post and found it helpful, share it with your friends. It might be the different point of view they need. Thank you!


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Ines Subashka

Инес Субашка е основател на IFS - зали за кондиционни тренировки и мобилност. Автор е на 6 книги за здравословно хранене и движение.

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