I copied this from a pamphlet I picked up at Agway this morning. It is called "A Guide to Growing Small Flocks."
Lighting
Lighting is of major importance in the well-being and overall health and performance of chickens. As each flock may have different requirements based on breed, type, season, housing, etc., it is best to review your individual needs with your chick supplier. Some general applications of lighting are as follows:
Broilers -
-- 24 hours of light for the first 3 days
-- 23 hours of light and 1 hour of darkness thereafter
-- 12 hours of light is sufficient under natural conditions to provide adequate feeding times
For the first week high intensity is recommended, i.e. 60-watt bulbs. Reducing lighting to low intensity (25 watt/15 watt bulbs) after the first week will help in controlling feather picking, improve feed efficiencey, and reduce bird activity.
Chicks for Egg Production -
The growing of pullets for egg production requires a different lighting program to achieve the best performance. There are many variations of acceptable lighting programs. For example: Three days of 24 hour light and then reduce the day length between days three and fifteen to 15 hours per day. From three weeks to 18 weeks reduce day length to 12 hours per day.
At three weeks of age, the light intensity can be dimmed to very low light.
It should be noted that a lighting program may not be feasible for small flocks of chicks.
The reason for and effects of light are:
-- Seasonal increasing day length during growing stimulates early body maturity and early egg production.
-- Seasonal decreasing day length slows growing maturity and is ideal for the best performance later on. Seasonal decreasing day length during egg production causes hens to begin the molting process.
Hope this helps.