Why Is the Summer Solstice the Healthiest Day of the Year?

Source: SubbotinaAnna Shutterstock

June 20, the summer solstice, offers the greatest opportunity to take advantage of the healing properties of sunlight because it is the longest day of the year. It also has the fewest hours of darkness to illuminate with electric light before we retire to bed. Exposure to the harmful effects of evening blue-rich LED and fluorescent lights can be minimized.

The seasonal differences in Northern US states, and cities like Boston, Chicago, and Seattle, can be substantial. When the sun sets at 9 pm after 16 hours of daylight only one hour of electric light is required if you retire to bed at 10 pm. In contrast at the winter solstice on December 21st, when darkness sets in at 4 pm, a full six hours of evening electric light exposure is required, and there are only eight hours when you can go outside and benefit from the healing power of daylight.

As I explain in my book, The Light Doctor: Using Light to Boost Health, Improve Sleep, and Live Longer, the evidence for the healing power of natural daylight is substantial. Large-scale prospective studies show that people exposed to the most outdoor natural daylight are the healthiest and live the longest. Their rates of cardiovascular disease and diabetes are significantly less than those who spend most of their time indoors.

Equally, the adverse impact of blue-rich electric lights in the nocturnal hours after sunset is considerable. Those exposed to the most blue-rich light after sunset are the sickest and have the shortest life expectancy. Sky-blue wavelengths are the signals used by our circadian clocks to determine whether it is day or night. The use of lights at night, after the sun has set, that emit those blue wavelengths (including most LED lights in the market today) disrupts our circadian rhythms and increases the risks of obesity, diabetes, heart disease, and endocrine-sensitive cancers like breast and prostate cancer.

It takes very little light at night to have an adverse effect. A recent large-scale study of 85,000 people with 13 million hours of light sensor data and 670,000 person-years of follow-up, showed that even dim bedroom light (1-6 lux) increased the risk of a diabetes diagnosis by 35 percent and the risk increased by 73 percent with brighter bedroom lights.

Of course, on any day, there are other health factors to consider besides day length and night length. Too much hot sunlight can be harmful, but obtaining the daylight benefits for circadian clocks does not require the brightest and hottest sunlight. One or two hours of natural outdoor morning daylight before the sun's peak is the most effective way to synchronize and strengthen the circadian system. And you don't need to be under full sunlight – sitting, or walking in shaded areas delivers enough of the blue-rich daylight signal to maintain healthy circadian clocks.