XPA and skin neoplasm: In addition, the clock controls the expression of XPA, a rate-limiting enzyme in DNA excision repair, which is lowest during the night, indicating that DNA excision repair is less efficient at night.47,60 Daytime-restricted feeding decreases the expression of Xpa and reverses the daily rhythms in sensitivity to UVB-induced DNA damage.20 One of the consequences of this diurnal variation in UVB-induced DNA damage is that more skin tumors form in mice when UVB is applied at night than during the day.47