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 other articles in the series.

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 spring returns.

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 earlier article in our series, we learned that light shining on the back of the eye sends “timing signals” 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 turn on and off 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 mental disorders. These include bipolar disorder and atypical depression (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 in the brain that control mood, as animal studies show.

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

Another brain-imaging study found 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 mental disorders, 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, “seasonal affective disorder”.

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 aren’t very seasonal. But for those who are, seasonality has a genetic component. Relatives of people with seasonal affective disorder are more likely to also experience seasonality.

Seasonality is also more common in conditions such as bipolar disorder. 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 “activated” 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 many antidepressants. There is some evidence of seasonal changes in serotonin levels, potentially being lower in winter.

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

But the neuroscience of seasonality is a developing area and more research is needed 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 including self-harm, depressive and anxiety symptoms, and lower wellbeing. It is also associated with higher rates of mental disorders, 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 many other mental processes.

But people differ hugely in their sensitivity to light. 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 depression and bipolar disorder 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 at risk of mental health problems. These include people with a family history of mental health problems or people who are night owls (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.

Jacob Crouse, Research Fellow in Youth Mental Health, Brain and Mind Centre, University of Sydney; Emiliana Tonini, Postdoctoral Research Fellow, Brain and Mind Centre, University of Sydney, and Ian Hickie, Co-Director, Health and Policy, Brain and Mind Centre, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Obesity is linked to many common diseases, such as type 2 diabetes, heart disease, fatty liver disease and knee osteoarthritis.

Obesity is currently defined using a person’s body mass index, or BMI. This is calculated as weight (in kilograms) divided by the square of height (in metres). In people of European descent, the BMI for obesity is 30 kg/m² and over.

But the risk to health and wellbeing is not determined by weight – and therefore BMI – alone. We’ve been part of a global collaboration that has spent the past two years discussing how this should change. Today we publish how we think obesity should be defined and why.

As we outline in The Lancet, having a larger body shouldn’t mean you’re diagnosed with “clinical obesity”. Such a diagnosis should depend on the level and location of body fat – and whether there are associated health problems.

What’s wrong with BMI?

The risk of ill health depends on the relative percentage of fat, bone and muscle making up a person’s body weight, as well as where the fat is distributed.

Athletes with a relatively high muscle mass, for example, may have a higher BMI. Even when that athlete has a BMI over 30 kg/m², their higher weight is due to excess muscle rather than excess fatty tissue.

People who carry their excess fatty tissue around their waist are at greatest risk of the health problems associated with obesity.

Fat stored deep in the abdomen and around the internal organs can release damaging molecules into the blood. These can then cause problems in other parts of the body.

But BMI alone does not tell us whether a person has health problems related to excess body fat. People with excess body fat don’t always have a BMI over 30, meaning they are not investigated for health problems associated with excess body fat. This might occur in a very tall person or in someone who tends to store body fat in the abdomen but who is of a “healthy” weight.

On the other hand, others who aren’t athletes but have excess fat may have a high BMI but no associated health problems.

BMI is therefore an imperfect tool to help us diagnose obesity.

What is the new definition?

The goal of the Lancet Diabetes & Endocrinology Commission on the Definition and Diagnosis of Clinical Obesity was to develop an approach to this definition and diagnosis. The commission, established in 2022 and led from King’s College London, has brought together 56 experts on aspects of obesity, including people with lived experience.

The commission’s definition and new diagnostic criteria shifts the focus from BMI alone. It incorporates other measurements, such as waist circumference, to confirm an excess or unhealthy distribution of body fat.

We define two categories of obesity based on objective signs and symptoms of poor health due to excess body fat.

1. Clinical obesity

A person with clinical obesity has signs and symptoms of ongoing organ dysfunction and/or difficulty with day-to-day activities of daily living (such as bathing, going to the toilet or dressing).

There are 18 diagnostic criteria for clinical obesity in adults and 13 in children and adolescents. These include:

• breathlessness caused by the effect of obesity on the lungs
• obesity-induced heart failure
• raised blood pressure
• fatty liver disease
• abnormalities in bones and joints that limit movement in children.

2. Pre-clinical obesity

A person with pre-clinical obesity has high levels of body fat that are not causing any illness.

People with pre-clinical obesity do not have any evidence of reduced tissue or organ function due to obesity and can complete day-to-day activities unhindered.

However, people with pre-clinical obesity are generally at higher risk of developing diseases such as heart disease, some cancers and type 2 diabetes.

What does this mean for obesity treatment?

Clinical obesity is a disease requiring access to effective health care.

For those with clinical obesity, the focus of health care should be on improving the health problems caused by obesity. People should be offered evidence-based treatment options after discussion with their health-care practitioner.

Treatment will include management of obesity-associated complications and may include specific obesity treatment aiming at decreasing fat mass, such as:

• support for behaviour change around diet, physical activity, sleep and screen use
• obesity-management medications to reduce appetite, lower weight and improve health outcomes such as blood glucose (sugar) and blood pressure
• metabolic bariatric surgery to treat obesity or reduce weight-related health complications.

Should pre-clinical obesity be treated?

For those with pre-clinical obesity, health care should be about risk-reduction and prevention of health problems related to obesity.

This may require health counselling, including support for health behaviour change, and monitoring over time.

Depending on the person’s individual risk – such as a family history of disease, level of body fat and changes over time – they may opt for one of the obesity treatments above.

Distinguishing people who don’t have illness from those who already have ongoing illness will enable personalised approaches to obesity prevention, management and treatment with more appropriate and cost-effective allocation of resources.

What happens next?

These new criteria for the diagnosis of clinical obesity will need to be adopted into national and international clinical practice guidelines and a range of obesity strategies.

Once adopted, training health professionals and health service managers, and educating the general public, will be vital.

Reframing the narrative of obesity may help eradicate misconceptions that contribute to stigma, including making false assumptions about the health status of people in larger bodies. A better understanding of the biology and health effects of obesity should also mean people in larger bodies are not blamed for their condition.

People with obesity or who have larger bodies should expect personalised, evidence-based assessments and advice, free of stigma and blame.

Louise Baur, Professor, Discipline of Child and Adolescent Health, University of Sydney; John B. Dixon, Adjunct Professor, Iverson Health Innovation Research Institute, Swinburne University of Technology; Priya Sumithran, Head of the Obesity and Metabolic Medicine Group in the Department of Surgery, School of Translational Medicine, Monash University, and Wendy A. Brown, Professor and Chair, Monash University Department of Surgery, School of Translational Medicine, Alfred Health, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.