This is what Ed Rosenthal has to say about it.
At temperatures below 60° F (15° C), photosynthesis and plant metabolism slow, stopping growth as it waits for better conditions. As soon as the temperature rises, the plant resumes full functioning. When the temperature falls below 40° F (4° C), marijuana plants experience tissue damage and require about 24 hours of warmer conditions to resume growth. Young marijuana plants are somewhat tolerant of low temperatures; when outdoors, seedlings have been known to pierce snow cover without ill effect. But low temperatures during ripening, even just overnight, delay or prevent bud maturation. Some equatorial varieties stop growth after a few nights with temperatures below 40° F (4° C).