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This is the next article in our ‘Light and health’ series, where we look at how light affects our physical and mental health in sometimes surprising ways. Read .

It’s spring and you’ve probably noticed a change in when the Sun rises and sets. But have you also noticed a change in your mood?

We’ve known for a while that light plays a role in our wellbeing. Many of us tend to feel more positive when .

But for others, big changes in light, such as at the start of spring, can be tough. And for many, bright light at night can be a problem. Here’s what’s going on.

An ancient rhythm of light and mood

In an in our series, we learned that light shining on the back of the eye sends ” ” to the brain and the master clock of the circadian system. This clock coordinates our daily (circadian) rhythms.

“Clock genes” also regulate circadian rhythms. These genes control the timing of when many other genes during the 24-hour, light-dark cycle.

But how is this all linked with our mood and mental health?

Circadian rhythms can be disrupted. This can happen if there are problems with how the body clock develops or functions, or if someone is routinely exposed to bright light at night.

When circadian disruption happens, it increases the risk of certain . These include and (a type of depression when someone is extra sleepy and has problems with their energy and metabolism).

Light on the brain

Light may also affect circuits that control mood, as .

There’s evidence this happens in humans. A brain-imaging study showed exposure to bright light in the daytime while inside the scanner of a brain region involved in mood and alertness.

Another brain-imaging study a link between daily exposure to sunlight and how the neurotransmitter (or chemical messenger) serotonin binds to receptors in the brain. We see alterations in serotonin binding in several , including depression.

What happens when the seasons change?

Light can also affect mood and mental health as the seasons change. During autumn and winter, symptoms such as low mood and fatigue can develop. But often, once spring and summer come round, these symptoms go away. This is called “seasonality” or, when severe, ” “.

What is less well known is that for other people, the change to spring and summer (when there is more light) can also come with a change in mood and mental health. Some people experience increases in energy and the drive to be active. This is positive for some but can be seriously destabilising for others. This too is an example of seasonality.

Most people . But for those who are, seasonality has a . Relatives of people with seasonal affective disorder are more likely to also experience seasonality.

Seasonality is also more common in conditions such as . For many people with such conditions, the shift into shorter day-lengths during winter can trigger a depressive episode.

Counterintuitively, the longer day-lengths in spring and summer can also destabilise people with bipolar disorder into an ” ” state where energy and activity are in overdrive, and symptoms are harder to manage. So, seasonality can be serious.

Alexis Hutcheon, who experiences seasonality and helped write this article, told us:

[…] the season change is like preparing for battle – I never know what’s coming, and I rarely come out unscathed. I’ve experienced both hypomanic and depressive episodes triggered by the season change, but regardless of whether I’m on the ‘up’ or the ‘down’, the one constant is that I can’t sleep. To manage, I try to stick to a strict routine, tweak medication, maximise my exposure to light, and always stay tuned in to those subtle shifts in mood. It’s a time of heightened awareness and trying to stay one step ahead.

So what’s going on in the brain?

One explanation for what’s going on in the brain when mental health fluctuates with the change in seasons relates to the neurotransmitters serotonin and dopamine.

Serotonin helps regulate mood and is the target of . There is some evidence of seasonal changes in serotonin levels, potentially being lower .

Dopamine is a neurotransmitter involved in reward, motivation and movement, and is also a target of some . Levels of dopamine may also change with the .

But the neuroscience of seasonality is a developing area and more research to know what’s going on in the brain.

How about bright light at night?

We know exposure to bright light at night (for instance, if someone is up all night) can disturb someone’s circadian rhythms.

This type of circadian rhythm disturbance is associated with higher rates of symptoms self-harm, depressive and anxiety symptoms, and lower wellbeing. It is also associated with higher rates of , such as major depression, bipolar disorder, psychotic disorders and post-traumatic stress disorder (or PTSD).

Why is this? Bright light at night confuses and destabilises the body clock. It disrupts the rhythmic regulation of mood, cognition, appetite, metabolism and .

But people differ hugely in their . While still a hypothesis, people who are most sensitive to light may be the most vulnerable to body clock disturbances caused by bright light at night, which then leads to a higher risk of mental health problems.

Where to from here?

Learning about light will help people better manage their mental health conditions.

By encouraging people to better align their lives to the light-dark cycle (to stabilise their body clock) we may also help prevent conditions such as and emerging in the first place.

Healthy light behaviours – avoiding light at night and seeking light during the day – are good for everyone. But they might be especially helpful for people of mental health problems. These include people with a family history of mental health problems or people who are (late sleepers and late risers), who are more at risk of body clock disturbances.

Alexis Hutcheon has lived experience of a mental health condition and helped write this article.

If this article has raised issues for you, or if you’re concerned about someone you know, call Lifeline on 13 11 14.