Because the lights are on during the day phase, they will heat the grow room. When the lights are off, the grow space may be heated by using a heater. This can be a hot air heater or an oil radiator. Make sure the heater has enough capacity to heat the whole grow area. The heater is often switched by the switchboard. The switchboard often has a relay with a heatercontact which turns on when the lights are off. In this way the heater turns on automatically and the grow area will be at temperature during the night period as well.