Although the risks associated with
extended wear contact lenses are well-known, there has not been an adequate explanation of why sleeping with the
lenses on increases the risk of
infection. It is contended that the reduction in the available
oxygen caused by
contact lenses does not in itself explain the high rate of corneal
infection caused by wearing the
lenses while sleeping. This paper examines the contribution that the mechanism of sleep itself makes to the risk of
infection, based on patient studies in two optometric practices. The results show that the risk of
ocular infection, particularly corneal
ulcers, is substantially increased with overnight wear of
lenses. A review of the role of
oxygen deprivation, atmospheric pollution, bacterial survival strategies,
infection-resistant defenses during wakefulness, and patterns of sleep indicates that
oxygen deprivation of the cornea is not the only factor in the increased risk associated with wearing the
lenses while sleeping. It is suggested that the combination of the introduction of airborne
pollutants to the
contact lens during the day, the reduced
oxygen transmission through
contact lenses and closed lids, and the lack of eye and lid movement during sleep together create the conditions in which
bacterial infection is most likely to occur.