No, guppies don’t need light at night. Guppies need darkness to sleep and rest properly. Having the aquarium lights on all the time may result in poor sleep quality, sickness, and eventually, death.
Guppies need six to eight hours of sleep each day and thus, you should keep the lights off for the same amount of time. If the tank gets plenty of natural, indirect light, you only need to keep the artificial lights on for eight to ten hours a day.