Our sleep follows a particular pattern: We normally get tired in the evening and are awake during the day. But this pattern isn’t only based on daylight. It is mainly controlled by our body clock (“circadian rhythm”), which is very difficult to change on purpose. The fact that we get tired at regular intervals, and then need to sleep, is out of our control.
How does our “body clock” work?
Our body clock is found in the suprachiasmatic nucleus (SCN) of the hypothalamus – the part of the brain that regulates the autonomic nervous system. The SCN is made up of nerve cells that send electric signals at specific intervals. The intervals change over the course of the day, which also tells other areas of the brain that there is a change.
People’s body clocks aren't all the same, and the circadian rhythm is never exactly 24 hours. It is usually slightly longer. For instance, in lab experiments without any obvious signs of time (like the natural shift between day and night), people’s circadian rhythm settles at around 25 hours after a few days. Humans need daylight to be able to set their body clock to a 24-hour day.
The photoreceptor cells in our eyes tell our brain whether it’s light or dark outside. Light controls how much melatonin we produce. This hormone greatly influences our body clock.
How does melatonin affect our sleep?
Melatonin is a hormone that is produced in the pineal gland in the brain. It has a number of functions, but its main one is to help regulate our sleep. For example, not having enough melatonin in our body can lead to sleep problems. Shifts in the timing of melatonin production can cause insomnia disorders.
Melatonin is mainly produced and released into the bloodstream between 9 p.m. and 8 a.m. – after that, the level quickly drops again. Darkness triggers the production, and daylight reduces it. So the number of daylight hours determines how long our body makes melatonin for. This means that our sleep patterns are also linked to the seasons.
High levels of melatonin make us feel sleepy and less able to concentrate. But the amount produced decreases as we age, which is why older people tend to sleep less.
What happens when you have jet lag?
If you take a long-haul flight across several time zones, the time when you arrive will be different to the time in the country you traveled from. This makes your body clock fall out of sync, and for a while you feel tired at the “wrong” time. But your melatonin levels are still controlled by daylight so your body clock gradually adjusts. This can take quite a few days. A similar thing often happens to shift workers when they change from a day shift to a night shift. It is best to avoid changing shifts on a frequent basis as it can have a serious impact on your health.
Why does light disturb our sleep at night?
Bright light during the night can reduce the amount of melatonin released into the bloodstream. This makes us stay awake longer than we would naturally. The influence of light depends on its brightness and color, when we’re exposed to it and for how long. The more similar it is to natural daylight, the more it can disturb our sleep. When someone works at night, the change in light exposure can disturb their circadian rhythm. This can prevent them from getting enough sleep in their rest periods too.
Sitting in front of a bright screen before bedtime can make it difficult to fall asleep and reduce the quality of your sleep. The "blue light" from smartphones, tablets and larger screens reduces melatonin production.
Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.
Menche N. Biologie Anatomie Physiologie. München: Urban und Fischer; 2016.
Pschyrembel. Klinisches Wörterbuch. Berlin: De Gruyter; 2017.
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