Studies (Mammoth Cave experiment of Kleitman) have shown that isolated individuals during several weeks of the 24-hour light and dark environmental cycle continue to maintain a rhythm in which sleep and waking state alternate over a period of approximately 24 hours. This suggests that the human being has an internal biological clock that saves its rhythm.

And it is also well known that the sleep-waking rhythm schedule of an individual can shift if the individual is exposed to a different light/darkness cycle schedule. This suggests that during the day, the light allows synchronization of your own rhythm with the terrestrial clock.

That’s why moving from several time zones will cause you jet lag. On one side you have your own internal clock that saved the old rhythm and on the other side, the light of your new zone no longer matches your old rhythm. The light/darkness alternation will force your body to resynchronize its circadian rhythm and this leads to the triggering of many biological processes.

Several biological phenomena depend on your circadian rhythms such as:

• Body temperature
• Hormones (cortisol, melatonin)
• Pulse rate and blood pressure
• The level of awakening
• Mood
• Vigilance
• Memory
• Intestinal activity

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Applied to sleep, the circadian rhythm makes you have powerful physiological signals that promote wakefulness and sleep at specific times of the day. In other words, your sleep synchronizes with the earth's cycle of light and darkness. This is why night workers tend to have more sleep troubles.

What you should care about the relation of the circadian rhythm and your sleep quality:

• Nocturnal exposure to artificial light is bad. It will influence and disturb your internal clock. 
  

• When your sleep is out of order it is important to reset it by exposing yourself to the light (preferably of the sunlight).
  

• Going to bed and always getting up at the same time will strengthen your internal clock and will allow you to fall asleep faster and have a better quality of sleep.

A little tip: When you come back very late after a night out, do not close your windows shutters if you have decided to sleep late. It’s a big mistake even if you do not want to be bothered by the light of the morning. By plunging yourself completely into the darkness when it is supposed to be the day, your body will not detect daylight in the morning and this will disrupt your biological clock. Indeed your body will act as if you have traveled to a different time zone. Consequently, your biological clock will try to adapt to this change and you will suffer from jetlag the next nights. You will probably not fall asleep at the usual time but much later because of that.

Scientific sources:

• Sleep and Wakefulness of Kleitman, Nathaniel. Chicago, IL: The University of Chicago Press. 1939.

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