Getting a daily fix of sunshine could boost your general health, with new research linking time spent outside with better mood, improved sleep and a lower lifetime risk of depression.
"Getting bright light in the day is as important as avoiding light at night," says psychologist and sleep researcher Sean Cain of Monash University in Melbourne, Australia, whose previous experimental studies have shown how artificial light impacts sleep and circadian rhythms.
In this new observational study, Cain and colleagues looked at the effect outdoor light exposure had on sleep and moods in over 400,000 people in the UK Biobank, a large study of UK adults that collects information on everything from exercise and sleep habits to medical diagnoses and health outcomes.
People had been asked about their mood, medications, and time spent outdoors on a typical day in summer and winter, amongst other things.
On average, UK adults in the study reported spending about 2.5 daylight hours outdoors, and early birds and morning people generally spent more time outside than night owls.
Past research has shown that spending time outdoors and in nature has a host of health benefits, part of which might be related to natural light being the most important environmental time cue for the body's circadian rhythms.
Not getting enough of natural light could be a key factor contributing to low mood and sleep troubles which are also associated with depression, a common mood disorder and one of the leading causes of disability worldwide.
"Humans evolved in an environment with a clear distinction between day and night, but our modern environments have blurred this distinction," the group explains in their paper.
These days, people spend "most waking hours in intermediate, artificial lighting conditions, due to reduced sunlight exposure and relatively bright night-time light exposure."
This can lead to disrupted sleep because light suppresses melatonin, a sleep-promoting hormone. Previous studies from Cain and team have found that nearly half of homes in a Melbourne study had bright enough light to suppress melatonin by 50 percent, though individual sensitivities to artificial light vary dramatically.
In this latest study, Cain and colleagues set about characterizing how the amount of daylight hours spent outside relates to mood, sleep, and health outcomes, something that has been studied less than the negative impacts of light at night.
Getting more light anytime between dawn and dusk was associated with better mood and improved sleep, as well as lower risk of depression and less use of antidepressant medications, the analysis showed.
Every additional hour of natural light was also linked to lower lifetime odds of depression, less antidepressant usage, and greater happiness. And those who reported better moods and sleep with more outdoor light tended to do so again the second time they were surveyed, on average four years later.
Modelling the data in this way – for a subset of some 20,000 people – allowed the researchers to assess the effect that earlier time spent outdoors had on later mood and sleep outcomes, while controlling for stable personal habits.
They also adjusted for seasonal differences, employment status, exercise, social activities and amount of sleep – all things which can impact mental health.
The sunny results are somewhat expected, based on what we know about light, nature, sleep patterns and moods, but what's encouraging to see is such a sizeable study demonstrating the effects of spending more time outdoors.
That said, being an observational study that relies on people answering questions about their daily habits and health, there may be differences between people's actual and reported behavior. And, while this research suggests getting outside could help to boost mood and improve sleep, that's not so easy for everyone to do.
Soaking up some sun in daylight hours is challenging for shift workers who are working against normal circadian rhythms. Waking up early before work might not be ideal for night owls and people with other chronotypes, either.
Some research suggests that defying your natural body clock is not so good for mental health, according to another recent study analysing UK Biobank data, which found people who were misaligned from their natural body clock were more likely to report depression and have lower wellbeing.
What is interesting though, is the parallel between the study findings on depression risk, antidepressant use, and the growing body of evidence suggesting that light therapy is an effective but underutilized therapy for treating depression, especially in combination with medication.
The study was published in the Journal of Affective Disorders.