Asher, A., Agricultural Research Organization, Newe Ya'ar, Ramat Yishay, Israel, Department of Evolutionary and Environmental Biology, Israeli Center for Interdisciplinary Research in Chronobiology, University of Haifa, Mount-Carmel, Haifa, Israel
Shabtay, A., Agricultural Research Organization, Newe Ya'ar, Ramat Yishay, Israel
Brosh, A., Agricultural Research Organization, Newe Ya'ar, Ramat Yishay, Israel
Eitam, H., Agricultural Research Organization, Newe Ya'ar, Ramat Yishay, Israel
Agmon, R., Agricultural Research Organization, Newe Ya'ar, Ramat Yishay, Israel
Cohen-Zinder, M., Agricultural Research Organization, Newe Ya'ar, Ramat Yishay, Israel
Zubidat, A.E., Department of Evolutionary and Environmental Biology, Israeli Center for Interdisciplinary Research in Chronobiology, University of Haifa, Mount-Carmel, Haifa, Israel
Haim, A., Department of Evolutionary and Environmental Biology, Israeli Center for Interdisciplinary Research in Chronobiology, University of Haifa, Mount-Carmel, Haifa, Israel

Pineal melatonin (MLT) is produced at highest levels during the night, under dark conditions. We evaluated differences in MLT-concentration by comparing daytime versus night time milk samples, from two dairy farms with different night illumination conditions: (1) natural dark (Dark-Night); (2) short wavelength Artificial Light at Night (ALAN, Night-Illuminated). Samples were collected from 14 Israeli Holstein cows from each commercial dairy farm at 04:30 h ("Night-milk") 12:30 h ("Day-milk") and analyzed for MLT-concentration. In order to study the effects of night illumination conditions on cows circadian rhythms, Heart Rate (HR) daily rhythms were recorded. MLT-concentrations of Night-milk samples from the dark-night group were significantly (p < 0.001) higher than those of Night-illuminated conditions (30.70 ± 1.79 and 17.81 ± 0.33 pg/ml, respectively). Interestingly, night illumination conditions also affected melatonin concentrations at daytime where under Dark-Night conditions values are significantly (p < 0.001) higher than Night-Illuminated conditions, (5.36 ± 0.33 and 3.30 ± 0.18 pg/ml, respectively). There were no significant differences between the two treatments in the milk yield and milk composition except somatic cell count (SCC), which was significantly lower (p = 0.02) in the Dark-Night group compared with the Night-Illuminated group. Cows in both groups presented a significant (p < 0.01) HR daily rhythm, therefore we assume that in the night illuminated cows feeding and milking time are the "time keeper", while in the Dark-night cows, HR rhythms were entrained by the light/dark cycle. The higher MLT-concentration in Dark-night cows with the lower SCC values calls upon farmers to avoid exposure of cows to ALAN. Therefore, under Dark-night conditions milk quality will improve by lowering SCC values where separation between night and day of such milk can produce chrono-functional milk, naturally rich with MLT. © 2015 Taylor & Francis Group, LLC.