There are anecdotal reports that looking good makes you feel good as well. One way to test this hypothesis would be to investigate the effects of spray tanning on mental health, such as depression.
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Mental health issues seem to be quite high on the governments agenda at the moment. It would therefore seem sensible to look into more possible solutions to the problem. Since spray tanning is a quick, easy, relatively cheap and appropriate for both genders this could go some way to improving the mental health of the nation should the research turn out to be positive.