Biorhythm and biological clock

Seeing light affects your biological clock. A small group of nerve cells regulates sleep with the help of light.
The biological clock is in your head. It is is no bigger than the period at the end of this sentence. 

It is located in the suprachiasmatic nucleus (SCN) that lies just behind the optic nerve.
The pineal gland produces the hormone melatonin that is very important for the sleep-wake rhythm.

Melatonin production is stimulated when it gets dark. Light stimuli inhibit melatonin production.
Exposure to bright light late in the evening therefore disrupts the ability to fall asleep properly. Conversely, a completely darkened room prevents you from waking up properly in the morning.
We can use this fact to get yourself used to summertime (daylight saving time) or wintertime by taking measures a week in advance.

Tips for switching to summer time (daylight saving time)
Get used to having your meals a little earlier until eventually an hour earlier.
Avoid blue light (screens) and LED lights.
In the morning, open the curtains immediately and look into the light! To switch to summertime, in spring, the clock goes forward an hour. The clock goes from 2 a.m. to 3 a.m.
People with brain injuries have a little more difficulty switching to summer time because we are more in our natural rhythm in the winter.

Tips for switching to winter time

Because your biological clock, your sleep-wake rhythm, is influenced by light, you can play with it a bit.
You shift the biological clock very gently if you take in a bit more light in the evening and don't turn on the lights or look at your screen in the morning. If you don't open the curtains right away, but keep it a bit dim for an hour, the biological clock gets used to it and shifts to a later time.
A week in advance, eat a quarter of an hour later every day and go to bed a bit later. Go to bed an hour later the night before winter time starts, so that you wake up on Sunday at your 'normal' time.

To switch to wintertime, the clock goes backward an hour. The clock goes from 3 a.m. to 2 a.m.

In the winter, we are more in our natural rhythm.

Different time zones and night shifts

Anyone who has ever traveled knows that jet lag can occur because you live in a different time zone.
Jet lag is a disruption of the sleep-wake rhythm.
People who have to work during the night also experience such a disruption of the biological clock. This is the case the first night(s)
and also when they have to switch back to daytime life.

In Europe, voices are being raised to abolish the switch in spring to summertime (daylight saving time).

Too many people lose their sleep-wake rhythm for a period.

People who are sensitive to this are, for example, people with brain damage. Babies and small children are also known to have difficulty switching to summer time, and to a lesser extent, winter time.

The biological clock also changes something in the body temperature. It is slightly higher in the evening and drops during the night. In the morning it is at its lowest. If you are too warm, you cannot sleep well. But if you have cold feet, you cannot fall asleep. Digestion also goes to a lower level at night.