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The CDC (and other sources) say that 20 minutes or so if sunlight right when you wake up resets your circadian rhythm, which helps with sleep, etc.

Does the light need to hit your eye or your skin?

i.e, Is this via seeing the light (eye, receptors in your eyes) or on your skin? or both.If its cloudy and I want to get more effect, should I expose more skin?

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Circadian rhythm maintenance begins with intrinsically photosensitive retinal ganglion cells (ipRGC's). In the retina, you have the traditional rod and cone cells that are responsible for vision. However, you also have ipRGC's that are sensitive to blue light. These cells connect to the suprachiasmatic nucleus in the hypothalamus. Every day, when your ipRGC's stop receiving blue light, your suprachiasmatic nucleus receives less stimulation and it signals the pineal gland to begin secreting melatonin, the sleep hormone, and a little while later you sleep. After some time, your pineal gland stops secreting melatonin and you wake back up.

While you're awake, blue light from sunlight stimulates your hypothalamus NOT to stimulate your pineal gland to release melatonin so you don't get sleepy. This blue light input sets your body's circadian rhythm naturally every day.

There is enough blue light in cloudy weather to set your circadian rhythm. However, certain conditions like Seasonal Affective Disorder (SAD) can leave certain people feeling crummy when the days get shorter at the end of the year. As a result, you can buy light therapy lamps to increase your blue light stimulation if you feel your environmental conditions aren't keeping your circadian rhythm where you'd like it to be.

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    Excellent detailed answer. We'd only ask that you edit it with some supporting references pro forma. Welcome to the site Asklepian. Sep 22, 2022 at 13:03
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From the CDC The circadian rhythm is reset from the eye.

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    You could improve this answer by adding the medically correct term suprachiasmic nucleus, as "circadian clock" is a non-medical term and this is a medical site. Aug 2, 2022 at 7:36
  • That is interesting @JiminyCricket. when the CDC themselves used the term circadian clock Sep 24, 2022 at 6:12
  • My apologies @ChrisRogers and OP. The terms I'm familiar with here in the UK seem to be at odds with the CDC's public-facing ones. Sep 24, 2022 at 6:36

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