It’s the same fluorescence phenomenon as the Day-Glo colors.
Most laundry detergents contain “brighteners” that absorb ultraviolet radiation from daylight and re-emit the energy as a bluish light that makes the shirt look “whiter and brighter.”
Moreover, the blue cancels out any yellowish cast. When stimulated by an ultraviolet lamp, which is even richer in ultraviolet radiation than daylight is, the fluorescence becomes bright enough to see as an actual glow-in-the-dark luminescence.
