The BMA have published a handy one stop shop to point people to when they insist on drivelling on about how you only use 10% of your brain, that you have to drink 8 glasses of water a day, that eating in the evening makes you fat, or that reading in low light makes you blind.
http://www.bmj.com/cgi/content/full/337/dec17_2/a2769
http://www.bmj.com/cgi/content/full/335/7633/1288
See also the nonsense of Detox: http://news.bbc.co.uk/1/hi/health/7808348.stm
Well done Science kids!