When you start to dig into the relationship between mood, sleep and the effects of light at different times of the day, you might start dozing at the complexity of it all.

When our lives were ruled by sunrise and sunset, our bodies were tuned to nod off in darkness, and wake up in light.

Without electricity, the body’s circadian clock was reliable as a well-kept fob watch.

Our lives may not have been free of depression, but a lack of sleep was a less common driver.

How sunlight helps our mood

Much of the conversation about sunlight, darkness, and mood is framed as a hormonal phenomenon.

Exposure to sunlight prods the brain to release serotonin, the hormone associated with boosting and balancing mood.

At night, the darker ambience triggers the brain to release melatonin, the hormone responsible for sleep.

Low levels of serotonin are associated with a higher risk of major depression triggered by a changing of the seasons.

What’s missing in this discussion is the active role of sleep.

A kick-on effect

A new study led by investigators from the Harvard-affiliated Brigham and Women’s Hospital found there is a three-way knock-on effect between sunlight, or its artificial equivalent, sleep, and lower odds of mild or severe depression triggers better sleep.

The study included more than 6600 participants.

Those who spent more time in bright light “had more regular sleep, and that more regular sleep was associated with lower depression symptoms”.

“Getting consistent, regular sleep has wide-ranging effects on our health,” said co-author Susan Redline, Professor of Sleep Medicine at Harvard Medical School and a senior physician in the Brigham’s Division of Sleep and Circadian Disorders.

“Future studies examining bright light therapy should not overlook the role sleep regularity may be playing in influencing mood and depression symptoms.”

To read more about the study and its limitations, see here.

Previous studies

A 2019 study suggested that at least 30 minutes of exposure to natural sunlight has a significant impact on the body’s internal clock, promoting more restful sleep.

A small 2022 study found that daytime bright light exposure can improve sleep quality and duration.

Healthline advises: If safe daily sunlight exposure isn’t possible, it’s worth investing in an artificial bright light device or bulbs.

A 2017 study of more than 100 office workers found “that those who were exposed to a lot of light in the morning had less time to fall asleep at night, especially in winter”.

They also found better synchronisation of the body’s internal clock and an improved quality of sleep.

How much sunlight do we need?

This is a vexing question. Especially in Australia which has big differences in sun intensity at the northern and southern reaches of the country.

Skin cancer is the most common cancer diagnosed in Australia.

About two in three Australians will be diagnosed with some form of skin cancer before the age of 70.

Sunlight dosage is more commonly discussed in terms of vitamin D, and we’ll use that as a proxy.

More than one in every three Australian adults has mild, moderate or severe vitamin D deficiency. Sunlight is the most efficient way of getting what you need.

Summer is the dangerous time for sunlight.

The Cancer Council advises that in summer, when the UV Index is 3 or above, a few minutes of mid-morning or mid-afternoon sun exposure to arms and hands on most days of the week should be sufficient to maintain adequate Vitamin D. 

In late autumn and winter, if the UV is below 3, sun protection is not necessary. That’s unless you’re near snow or other reflective surfaces.

See more here.