The findings are based on a year long study of patients attending the Royal Eye Hospital in Manchester, UK, with acute eye problems.
All those wearing contact lenses were asked to supply details of lens hygiene, type, and pattern of wear, including whether they slept in them.
Specialists scored eye problems on the cornea according to their severity, a score above 8 denoting severe keratitis. Eighty were scored below 8, defined as 'non-severe keratitis,' and 38 were scored above.
Among the 80 who scored below 8, 18 slept in their lenses. Among the 38 with severe keratitis, 9 did so.
Four types of lenses were studied - rigid, hydrogel daily disposable, hydrogel, and silicone hydrogel. There were no differences in risk of severe keratitis between the different types of lenses when they were worn during the day.
But the risk of severe keratitis rose when lenses were slept in, and there were some differences between lens types.
Rates were 96 per 10,000 wearers a year for hydrogel lenses, compared with almost 20 per 10,000 wearers a year for silicone hydrogel lenses - a fivefold difference.
Almost 3 million people wear contact lenses in the UK, around 55,000 of whom live in the catchment area served by the hospital, including 30,000 hydrogel lens wearers and 1700 silicone hydrogel lens wearers.
But those wearing hydrogel lenses while they slept were five times more likely to develop severe keratitis than those wearing silicone hydrogel lenses.
"Those who choose to sleep in lenses should be advised to wear silicone hydrogel lenses, which carry a five times decreased risk of severe keratitis for extended wear compared with hydrogel lenses," conclude the authors.
British Journal of Ophthalmology