Using a UV flashlight is also recommended when sunlight isn't available.
Not recommended, but last resort, you can charge SOME of your Glow items using an LED light. This only works for Invisible Glow colors and a couple of easy to charge colors. This will be better explained below. As this is not the suggested way of charging, it may take several minutes to charge this way and it usually does not last long.
(Please read paragraph below that talks about the different colors, their brightness and longevity)