i googled this, to find out what was out there on the subject. i found this very good article –

We’ve all heard the saying ‘laughter is the best medicine’ and like most of these sayings it has some truth in it. The positive effects of laughter and a sense of humor cover not only the psychological but also the physical and the lifestyle aspects too. Humor  and laughter are infectious and can bring people together and make them forget their troubles and one of the best tools to maintain a positive outlook and good health.

Laughter has an obvious and powerful effect on the body – and is not only a result of happiness and humor but also a cause of it – making it self sustaining (which is only a bad thing if your at a funeral or in a lecture). These positive effects of laughter can be explained by the fact that it triggers the release of endorphins and serotonin which are the feel good hormones that create feelings of euphoria, love and happiness. Laughter also works through a psychological phenomenon known as ‘facial feedback’ whereby pulling a certain expression can actually cause you to experience the corresponding emotion. Thereby the simple act of smiling from laughing can raise your mood. Finally, the fact that laughter is contagious means that you can almost ‘contaminate’ yourself, causing an endless cycle of laughter that eventually leaves you doubled over and struggling to catch your breath… As well as triggering endorphins however, laughter also suppresses stress hormones to further increase your good mood.

Furthermore though, there are also some physical and very positive effects of laughter than actual result in internal changes to your body. Firstly, laughter relaxes the body by overriding stress and anger so relieving tension – an effect that can last for up to 45 minutes after. One area that doesn’t relax however is the stomach, which works overtime to cause your hyena like belly laugh (which would be where the term comes from). If you’re looking to sculpt yourself a six pack then, laughter may just be the most fun way to do so. The aforementioned endorphins can also suppress pain making laughter a great pain killer as well as a great antidepressant.

The positive effects of laughter also extend to health benefits that can prevent serious illness. This includes an improved immune system which it achieves by increasing immune cells and antibodies to strengthen its ability to cope with viruses and disease. This effect augments the lack of stress hormone which also provides a boost to the immune system and makes laughter a viable form of combating a variety of viruses and almost antibiotic-like in its effect. Laughter also protects the heart by improving the function of blood vessels, encouraging blood flow and over time improving cholesterol and blood pressure thereby helping to prevent heart attack, heart disease, or other circulatory system diseases.

Laughter is also correlated with mental health and has many social and interpersonal benefits. With all these benefits, laughter is an antidepressant, pain killer, workout, stress reliever and a great way to make friends and strengthen bonds. And best of all – it’s free!

Source: Health Guidance