Previous studies indicate that milk yield can be increased by the provision of supplementary light when natural daylength declines to between 8 and 10 hours per day. However, the response has been shown to be variable and the mechanism by which milk production is stimulated remains to be determined. One theory is that photoperiod could directly influence the hedonistic response of cows to their environment and hence improve welfare by reducing stress. Increasing production by manipulating photoperiod is more acceptable to the consumer than the administration of exogenous hormones, although the commercial benefits require further investigation. The objective of this study was to investigate the effects of providing supplementary light for dairy cows during the spring months when natural daylength is increasing, most previous studies having compared light supplemented cows with those in declining daylength.