A study involving the effects of supplementary artificial lighting during the winter months upon a line of Brown Leghorn hens, reveals that the onset of the annual rest is delayed as a result of its use, and the tendency for onset of rest to become earlier in successive laying cycles is reduced.

The influence of time of onset in determining the duration of rest is higher in birds that commence their rest early than in those commencing later, resulting in a curvilinear relationship between these two parameters. A hypothesis accounting for this phenomenon is proposed.

The relative importance of age at onset of first annual rest in determining its duration appears to be negligible. This conclusion is based on an analysis of records pertaining to a generation of the same line which was hatched over an abnormally long hatching season, thus enabling the effects of “date” and “age” to be separated.