Diurnal rhythm in the human rod ERG. Relationship to cyclic lighting.

Five normal subjects were monitored with periodic full-field ERG testing during entrainment to a 14L:10D lighting cycle. On the second day of entrainment, mean ERG threshold was significantly higher 1.5 hr after light onset than at other times of day and was comparable to a previously reported threshold elevation following 7 days of entrainment. This diurnal rhythm was not detectable in eyes entrained for 3 days that were then maintained in darkness for 1 day. The findings support the idea that 2 days of entrainment are required to maximize the diurnal rhythm in the human rod ERG and that an adapting light may be required to trigger this rhythm.