If you're wearing contacts, you might want to take a look at this.

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Sources:

http://www.mirror.co.uk/news/uk-news/student-who-ripped-cornea-after-9161849
http://jezebel.com/student-keeps-contacts-in-for-six-months-amoeba-eats-1604319154
http://www.med.umich.edu/1libr/Ophthalmology/ContactLens/SignsofInfectionfromContactLenses.pdf
http://www.allaboutvision.com/contacts/faq/cls-damage-eyes.htm
http://www.allaboutvision.com/contacts/extended.htm
http://www.ccmr.cornell.edu/faqs/why-are-eyeballs-wet/
http://www.masseyeandear.org/for-patients/patient-guide/patient-education/diseases-and-conditions/cornea/function
http://www.d.umn.edu/~jfitzake/Lectures/DMED/Vision/Optics/Crying.html
https://www.aao.org/eyenet/article/confronting-corneal-ulcers?july-2012
https://u.osu.edu/buckmdblog/2013/02/19/how-long-can-i-leave-my-contact-lenses-in/
http://www.aaojournal.org/article/S0161-6420(99)00027-5/fulltext
https://www.ncbi.nlm.nih.gov/pubmed/12394550
https://www.ncbi.nlm.nih.gov/pubmed/3863808
https://www.ncbi.nlm.nih.gov/books/NBK234044/