Grow light timers allow the gardener to set up a schedule that encourages maximum productivity and health. Although plants can grow very quickly under 24 hours of light, they typically need a minimum amount of darkness in order to trigger flowering or fruiting. Gardeners can choose from mechanical and digital timers. Mechanical timers are usually cheaper, and can be set for one photoperiod within a 24 hour cycle. Although more expensive, digital timers allow the gardener to program several cycles of light per day. Timers can also be used to control fans, pumps, or motor relays.