Timing of oviposition in mixed systems using bright light, dim light and darkness.

1. Two experiments were conducted to examine time of oviposition for hens exposed to continuous dim lighting, to dim lighting alternating with bright lighting in a 24 h cycle or to a mixed system using bright light, dim light and darkness. 2. Under continuous dim lighting (0.3 lux), the pattern of ovipositions was the same as that reported previously for constant darkness, more eggs being laid around midnight than around noon. 3. With alternating bright and dim phases, mean time of lay was approximately 16 h after the transition from bright to dim lighting, which was 3 h earlier than under the corresponding cycle of light and dark. This phase advance was the same whether the bright:dim ratio was 16:1 or 160:1. 4. Dim lighting (1.25 lux) preceded by a period of normal lighting (5 lux or 50 lux) and followed by 8 h darkness was treated as part of the photoperiod. 5. It is concluded that, when there is no darkness, a period of dim lighting is treated as darkness, provided the contrast between bright and dim phases is sufficient. However, when darkness, dim light and bright light are all included in a cycle, the dim light is treated as part of the photoperiod, even though there may be a contrast between the brightly lit and dimly lit phases which, in the absence of darkness, would cause the dim phase to be treated as dark.