Science has known for decades that exposure to sunlight has something to do with maintaining a healthy (strong) circadian rhythm.
Only in the last few years however, has science found out that sunlight when the sun is high in the sky has very little effect on the circadian rhythm: the effect is mediated by two classes of receptors in the retina, one sensitive mostly to blue light, the other sensitive mostly to yellow light, and sunlight only affects the circadian rhythm when the ratio of the activation of the two classes of receptors is what is produced when the sun is low in the sky.
But at least among my acquaintances, the recent knowledge is unknown—maybe partly because people think they already know and have known for decades what they need to know about the effect of sunlight on the circadian rhythm.
If the sky is cloudless, only 5 minutes of exposure is sufficient. On a very overcast day, 20 minutes is needed. The aforementioned receptors are only in the half of the retina (namely, the bottom half) that gets light from the top half of the field of vision, but they are all over that half of the retina, so for example you wouldn’t want a brim of a hat obstructing your view of the sky because that could easily leave half of the relevant receptors in your retina unstimulated by light, which assuming the simplest model of the system, would tend to double (e.g., from 5 minutes to 10 minutes if the sky is cloudless) the duration of exposure to sunlight necessary to have the desired effect (on your circadian rhythm).
I take my morning walk right around sunrise, at which time the sun is almost always obscured by some house or another with result that almost all the light entering my eyes is “indirect” sunlight (scattered by the atmosphere). (In coastal California, the houses are large, and separated by only ten feet or so from each other.) But this “indirect” sunlight is in my experience at least as effective as direct sunlight is at keeping my bed time nice and early in the night.
The sky at sunrise does not seem that bright, but that is basically an optical illusion whereby humans overestimate the brightness of point sources of light and underestimate the brightness of light spread over a significant fraction of one’s field of vision.
In addition to getting sun in the morning while the sun is still low in the sky, to maintain a good circadian rhythm it is important to avoid even dim light between the hours of 11 PM and 4 AM.
(I realize that getting to bed at a sufficiently early hour is only tangential to the OP’s question, but I couldn’t resist chiming in on Romeo’s comment.)
Science has known for decades that exposure to sunlight has something to do with maintaining a healthy (strong) circadian rhythm.
Only in the last few years however, has science found out that sunlight when the sun is high in the sky has very little effect on the circadian rhythm: the effect is mediated by two classes of receptors in the retina, one sensitive mostly to blue light, the other sensitive mostly to yellow light, and sunlight only affects the circadian rhythm when the ratio of the activation of the two classes of receptors is what is produced when the sun is low in the sky.
But at least among my acquaintances, the recent knowledge is unknown—maybe partly because people think they already know and have known for decades what they need to know about the effect of sunlight on the circadian rhythm.
If the sky is cloudless, only 5 minutes of exposure is sufficient. On a very overcast day, 20 minutes is needed. The aforementioned receptors are only in the half of the retina (namely, the bottom half) that gets light from the top half of the field of vision, but they are all over that half of the retina, so for example you wouldn’t want a brim of a hat obstructing your view of the sky because that could easily leave half of the relevant receptors in your retina unstimulated by light, which assuming the simplest model of the system, would tend to double (e.g., from 5 minutes to 10 minutes if the sky is cloudless) the duration of exposure to sunlight necessary to have the desired effect (on your circadian rhythm).
I take my morning walk right around sunrise, at which time the sun is almost always obscured by some house or another with result that almost all the light entering my eyes is “indirect” sunlight (scattered by the atmosphere). (In coastal California, the houses are large, and separated by only ten feet or so from each other.) But this “indirect” sunlight is in my experience at least as effective as direct sunlight is at keeping my bed time nice and early in the night.
The sky at sunrise does not seem that bright, but that is basically an optical illusion whereby humans overestimate the brightness of point sources of light and underestimate the brightness of light spread over a significant fraction of one’s field of vision.
In addition to getting sun in the morning while the sun is still low in the sky, to maintain a good circadian rhythm it is important to avoid even dim light between the hours of 11 PM and 4 AM.
(I realize that getting to bed at a sufficiently early hour is only tangential to the OP’s question, but I couldn’t resist chiming in on Romeo’s comment.)