Before you go to bed, try this exercise: Write down three things that went well today and why they did.

Or pen a letter to someone expressing gratitude for something he did.

Or, instead of watching TV programmes after dinner, take a walk around your neighbourhood to relax and clear the mind.

Such exercises to cultivate a positive mindset would have been dismissed by serious medical practitioners as pop psychology and happy-clappy talk.

That is changing.

Positive psychology is increasingly being embraced by mainstream medicine to prevent and treat mental illness, as research unfolds showing it can yield concrete gains in mental and physical health.

Psychologists and psychiatrists in at least three public hospitals - KK Women's and Children's Hospital (KKH), Khoo Teck Puat Hospital (KTPH) and Singapore General Hospital (SGH) - are steering patients diagnosed with anxiety and depression towards positivity-building exercises.

With its focus on cultivating inner strength and happiness, positive psychology is like the yin to traditional psychology's yang, said Ms Lynn Soh, a senior psychologist and head of psychology service at KKH.

One could say it is to 'build what's strong' in individuals, while traditional psychotherapy is used to 'fix what's wrong', in the words of Dr Martin Seligman, an American psychologist credited to be the father of the positive psychology movement.

Mental-health professionals here say positive psychology provides a much-needed balance to traditional psychology's focus on what makes people mentally unwell.

Psychologists at the Institute of Mental Health (IMH), however, do not use positive psychology techniques as their patients usually have serious mental illnesses, and such exercises are not as effective for them as for those with milder mental-health concerns or well people seeking to fulfil their potential.

But in other public hospitals, there is a rising number of psychologists and psychiatrists who use techniques such as three good things and signature strengths (see other story) in patients with milder mental health concerns.

For instance, most psychologists at KKH now use these techniques in three in five of their patients with mild to moderate anxiety and depression.

A few mental-health experts at KTPH and SGH also apply these techniques in their practice.

Ms Lisa Choo, a senior clinical psychologist at KTPH, uses positivity-building exercises in at least three in five of her patients with mild to moderate anxiety and depression, while SGH psychologist Tina George feels at least half her patients with these problems, as well as eating disorders, would benefit.

These exercises are often used alongside psychological techniques such as cognitive behavioural therapy (CBT), which are proven in treating symptoms of mental illnesses. CBT encourages people to challenge and change automatic negative thoughts and behaviour.

While such techniques can treat mental illnesses, they do not necessarily lead to a happier and more meaningful life, said Ms Stephanie Tan, a psychologist at KKH.

Happiness comes over and above the absence of mental illnesses, which could explain why even after undergoing CBT, some patients still feel a sense of emptiness, said Ms Tan. This is where positive psychology, with its emphasis on building inner strength, may come in useful.

In fact, there is early evidence that such 'core strengthening' could well mitigate a patient's risk of a relapse, even in the severely depressed.

A 2006 study on 46 severely depressed patients published in the American Psychologist found that those who had positive psychology exercises showed significantly more improvement in their symptoms and overall mental well-being after 14 sessions compared to those who had treatment as usual.

KTPH's department of psychological medicine is so optimistic about the benefits of positive psychology that it rolled out an eight-week programme for hospital employees to learn about it this year.

'By doing so, we hope to improve the well-being of our staff,' said Dr Christopher Cheok, a senior consultant psychiatrist and head of psychological medicine at KTPH.

He said: 'Caring for the ill is a meaningful but stressful career and we need to prevent burnout among staff.'

Next week has been earmarked by KTPH as Happiness Week, during which staff can practise incorporating positive psychology into their daily lives. Posters containing tips on how to do so will be put up at various parts of the hospital.

KTPH also plans to hold a public forum on the topic next February, in conjunction with World Happiness Day.

While there seems to be little downside to using positive psychology, the hospital does not rush to use this or psychotherapy for patients who are severely depressed.

Said Dr Cheok: 'We need to treat them till their symptoms improve and implement psychotherapy when they are able to process the information.'

The positive psychology interventions (PPIs) also need to be adapted to different cultural practices.

Said Dr Cheok: 'People who come from families who don't express much emotion may find it awkward to express gratitude openly. We have to discuss with the patient what he is comfortable with and can put into daily practice.'

Hospital psychologists and psychiatrists said patients have, on the whole, been receptive to PPIs.

To detractors who argue that one's personality is set, research suggests optimism can be practised.

A review of 51 positive psychology studies done between 1977 and 2008 on more than 4,000 people found that twice as many - or about 65 per cent - of those who had PPIs and traditional psychotherapy experienced gains in well-being and reduced depression compared to 35per cent of those who had only traditional psychotherapy. The study was published in the Journal Of Clinical Psychology in 2009.

Those who benefited included both the mentally well and the clinically depressed.

Positive psychology works to change people's interpretations of events from negative to positive so the same glass is seen as half full rather than half empty.

The benefits go beyond merely feeling good. Studies are beginning to show that positive emotions can even ease adverse physical effects of negative emotions, such as elevated heart rates and blood pressure.

Regular practice is key, said Ms Choo.

The 2009 review also found that those who are motivated to practise the exercises regularly and make them a habit show greater improvement in well-being and are less depressed.

Said Ms Choo: 'Being happy is not just a feeling that comes with the flip of a switch. If you want to be happy, you need to work on it.'

And there is nothing to lose.

Quoting University of Pennsylvania psychologist Angela Lee Duckworth, Ms Choo said: 'After all, there are few risks involved when one discovers his strengths or focuses on the positive side of life - and there may be valuable benefits.'