Just like an old clock, biological clocks must be adjusted to the correct time every day. Light is detected by cells at the back of our eyes, called photoreceptors. Most photoreceptors detect light so that we can see the world around us. But, in 2002, a new type of photoreceptor was discovered that sends signals directly to the SCN. These special photoreceptors are called intrinsic photosensitive retinal ganglion cells, or ipRGCs. If the ipRGCs are working, even blind people can keep their rhythms aligned with the sunlight.
Using sunlight, the SCN can adjust the circadian rhythm to gradual changes in daylight hours as we progress through the seasons. But sudden changes in the light-dark cycle can leave us feeling totally out of whack. You may have experienced this yourself: it is called jet lag. Since the invention of airplanes, humans have been able to cross time zones in a matter of hours. An airplane can dump us in bright daylight when our biological clocks are preparing us for sleep. This can leave us feeling drowsy, dizzy and even queasy.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00005