Daylength and breeding in the domestic cat.

A period of extended daylight (more than 12 hours per day) following one of short days induces oestrus. The longer the short-day period, the more pronounced the reaction. This change from short-day to long-day periods can be successfully repeated at least 6 times in a year. However, the number of litters produced per year is largest when a regular 12 h light/dark cycle is maintained.