Poorly designed websites can be harmful for your health, according to a new study.

Websites that are slow to load and badly designed often lead to an increase in the number of mouse clicks, increased heart rate and muscle strains, a study by the Social Issues Research Centre (SIRC) found.

The report states that some particularly bad websites can even be harmful for a person's health due to the stress reactions they cause.

A group of 2,500 web users took part in the study. Their brain activity, heart rate and muscle tensions were measured as they were surfing the web.

The results showed that websites that loaded slowly, and/or had a confusing and complicated navigation layout trigged a number of physiological reactions. Pop-up windows and advertisements also caused similar reactions.

Only a few of the sample group were able to keep their calm as they encountered the poor websites, as most got considerably stressed. The physical reactions were severe sweating, grinding of the teeth, and an increased heart rate.

Many users frantically clicked their mouse and moved the mouse indicator around the computer screen. Some users even screamed loudly at their computers.

In its test, the SIRC used a 'perfect' website that functioned well as the starting point. Then it subjected users to a number of websites with 'crazy graphics and slow-loading pages' and recorded the reactions.

Internet users want speed, function and accuracy from their web pages, the report said.