Daily rhythm of tear production in normal dog maintained under different Light/Dark cycles

This study was conduced to assess the daily rhythm of tear production in clinically healthy dog. For our study eight female purebred Beagles were subjected to three different Light/Dark schedules: 12/12 L/D, 24/0 L/D and 0/24 L/D cycles. In all subjects Schirmer tear test I was performed at 4 h intervals over a 24 h period. A statistical significant effect of photoperiod was observed comparing the three different L/D schedules, and a statistical significant difference was observed comparing left and right eye during the 12/12 L/D schedule. We demonstrated daily variation of tear production in dogs maintained under an L/D cycle. We also provided strong evidence that the rhythm of tear production is endogenously generated because it persisted in constant darkness. Although the range of excursion of the daily/circadian oscillation in STT I (about 2 mm/min) is likely too narrow to be of clinical significance, it is statistically significant and may have physiological implications not yet appreciated.