The effect of light exposure on mental and physical health has been highlighted by new Australian research that suggests antidepressants could be less effective for night owls than for early risers.

A study published in the medical journal Chronobiology International investigated responses to antidepressant medication in 1,000 people who were either "morning types" or "evening types" — that is, people who got up early or stayed up late.

It found a higher number of evening types reported the medication, known as selective serotonin reuptake inhibitors (SSRI), was not effective at reducing depressive symptoms and suicidal feelings.

They also switched medication more often.

Authors of the study, PhD candidate Elise McGlashan and Associate Professor Sean Cain from the Monash Institute of Cognitive and Clinical Neurosciences, said the times of day people were waking up and going to bed was a key factor.

"We believe that evening types tend to have more irregular schedules. And what these [antidepressant] drugs do is they boost how your body clock responds to light, so if you keep an irregular schedule that will be made worse," Dr Cain said.

"In general, evening types tend to be more prone to depression and more resistant to typical treatment. They're a highly vulnerable group."

Dr Cain said night owls tended to have more exposure to light at night and less in the morning, so their 24-hour body clock was out of whack.

"Recovering from depression and getting efficacy out of antidepressant medication has a lot to do with keeping a highly regular schedule, and really the most important aspect of that schedule is a regular light-dark schedule," he said.

The benefits of morning light exposure

The research also has implications for people without depression or mental health struggles.

There is growing evidence that shows exposure to artificial light at night — such as from phones, tablet devices and computers — delays the body clock and leads to poor sleep.

Morning light exposure helps regulate the body's internal 24-hour clock. ( Jo Joyce, ABC Open )

Poor sleep is linked to an array of health problems, including obesity, diabetes and mental health.

On the flip side, light in the morning can have a significant role in promoting better physical and mental health.

"The more light you get throughout the day would be associated with greater health benefits, but the other side of it is avoiding light at night," Dr Cain said.

"Your body is composed of thousands of clocks — you have a core biological clock in your brain, in your hypothalamus — but you also have clocks in other areas of your brain and throughout all the tissues of your body.

"When you have a strong external light signal it helps maintain healthy relationships amongst all of the different rhythms in your body.

"When you start, like evening types tend to do, to have irregular schedules and low morning light exposure, all of those internal clocks — the clocks in your heart, liver, pancreas and in your brain — they tend to get misaligned with each other, which leads to poorer health, poorer mood and poorer metabolism."

Dr Cain likens the way morning light can boost mood and alertness to caffeine.

Turn off LED lights and put down your phone at night

While experts have been pointing to the negative effects of evening screen time on sleep, Dr Cain says another offender is LED lights, which are now found in most homes.

Dr Cain recommends reducing phone use and turning off LED lights at night. ( ABC South East SA: Kate Hill )

The blue light they emit tricks your body into thinking it's still daytime, making you alert and reducing your levels of melatonin, the hormone that promotes sleep.

"I don't use LED lights in my home. I use incandescent light, and that helps you maintain better sleep and not disturb your rhythms because you're highly sensitive to light at night," Dr Cain said.

"Even if the light appears low or not very bright, it can have a huge effect on that internal clock."

Dr Cain said if people refused to reduce their phone and screen use at night they should at least use red-tinting apps like Nightshift or F.lux to reduce the amount of blue light emitted by their devices.

"I believe in 10 years time we're going to appreciate how much damage we've been doing to ourselves with phones at night," he said.

"We're also finding that disturbing one's rhythms also tends to lead to greater weight gain."

Keep your bedtime and alarm clock consistent

Julia Stone, a PhD candidate at Monash University, is researching the effects of shift work on circadian rhythm, and how light affects the body clock.

Shift workers have been shown to be more susceptible to obesity and poor food choices.

Irregular body clocks are linked to weight gain, poor sleep and low concentration. ( iStockPhoto )

But irregular sleep patterns, even in people who work nine to five and who might stay up late and sleep in on weekends, or who sometimes stay up past their bedtime during the week, can have far-reaching consequences.

"We know there are impacts on mental performance, on alertness, sleep problems, we see metabolic problems, increased incidence of some cancers — for instance, breast cancer," she said.

"Obviously if your eyes are closed and you're asleep, you're not going to be able to get that light, so when you're sleeping in for a weekend … we tend to see a phase delay, so people's body clocks are set later.

"So getting that morning light after a late night like that is going to be important to stabilise your body clock to keep it regular, and keep your sleep regular."