Fatty acid levels in milk vary between day and night milking. Many dairy cows are still kept under white light-emitting diode (W-LED) illumination throughout the night, although it is known to disrupt endogenous circadian rhythms. We investigated the effects of whole-night W-LED illumination (125 lux) on milk yield and circadian composition, compared to a natural light–dark (LD) cycle of 10 h light. Mid–late lactation cows (n = 34) that were exposed to natural LD cycle showed circadian variation in milk fat composition, characterized by higher health-promoting monounsaturated fatty acid (MUFA; 24.2 ± 0.4 vs. 23.2 ± 0.4 g/100 g fat, p < 0.001) and lower saturated fatty acid levels (71.2 ± 0.4 vs. 72.5 ± 0.4, p < 0.001) at 13:30 h (day milk) than at 03:30 h (night milk). Compared to natural LD (n = 16), W-LED (n = 18) did not affect milk production or milk fat yields, yet abolished the milking time variation in milk fat composition towards a less healthy fatty acid profile. This lowered MUFA levels of day milk (23.8 ± 0.4 vs. 26.7 ± 0.4, p < 0.01). Therefore, W-LED has no commercial advantage over the tested natural LD cycle, and conversely, even shows circadian disruption. Accordingly, a natural LD cycle of 10 h light is preferable over W-LED from the perspective of cost savings, the cows’ well-being, and preserving the natural milk fat profile, as the nutritional value of the day milk is slightly higher.

Fatty acid levels vary between day and night milking. Many dairy cows are kept under whole light at night (LAN) illumination using a white light-emitting diode (W-LED) suspected of disrupting circadian rhythms. We investigated the effect of W-LED LAN and same lux level red LED (R-LED) LAN, which is known not to disrupt circadian rhythms in other mammals, on milk yield and circadian composition, compared to a natural light-dark (LD)-cycle of 10-11 h light. Cows under the natural LD-cycle showed circadian variation in milk-fat composition, characterized by higher health-promoting monounsaturated fatty acid (MUFA) and lower saturated fatty acid (SFA) levels in 13:30-h (day milk, DM) than 03:30-h (night milk, NM). W-LED did not affect milk and milk-fat yields, yet abolished the circadian pattern of milk-fat composition towards a less healthy fatty acid profile by lowering MUFA levels of DM. Switching W-LED to R-LED reversed these circadian disruptions. Therefore, R-LED and W-LED have no commercial advantage over the tested LD-cycle, while W-LED is disadvantageous, showing a circadian disruption. Accordingly, if the illumination is indispensable, the R-LED regime is preferable over W-LED for cow’s wellbeing and for preserving the natural milk-fat profile; while differentiating DM from NM from a commercial-health-promoting perspective.