A stubborn idea follows moss around: that it is a plant of the dark, content in a dank cellar or under a stone where nothing else will grow. It is easy to see why, since moss does turn up in gloomy corners that defeat grass and flowers. The plant itself tells a different story. Moss is green, and green means it lives on light, so the real question is not whether it needs light but how much, and of what kind.
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It feeds on light like anything green
Every moss is packed with chlorophyll and makes its living by photosynthesis, building sugars from water, air and light much as an oak or a blade of grass does. Shut it away in a genuinely dark room and it will slowly starve, paling and thinning until it dies, however faithfully you mist it. What sets moss apart is not that it does without light but how little it can manage on. Plenty of species run quite happily on the dim, green, filtered glow of a woodland floor, levels far too low to keep a lawn grass alive, and that modest appetite is the key to almost everything moss does.
Why shade seems to be its home
Set that small light requirement beside the habits of bigger plants and the pattern falls out on its own. Grasses, seedlings and border flowers all want strong light and a decent root-run; deny them either and they sulk or fail. Moss wants neither, so in a shaded, damp, trodden spot where taller plants cannot get going, it simply moves into the space they have left empty. This is why a mossy lawn is really a message about the site rather than about the moss: too shaded, too wet, the soil too poor or too compacted for grass to hold its own. I read that message properly under why moss is growing in my lawn. The moss is not hungry for darkness; it is quietly winning a contest the grass has already lost.
The sun-baked mosses people forget
For all that, a fair number of mosses live in blazing, open places and would rot in the damp shade the woodland kinds prefer. Run your eye along the top of a sunny wall for the little green rosettes of wall screw-moss (Tortula muralis), or at the silvery tufts jammed into hot pavement cracks, or at the tight grey buns of Grimmia on a boulder that faces south all day. Star moss and the woolly fringe-mosses of bare mountain tops keep the same sun-hardy company. They cope by bowing out when the sun is fierce, drying to a crisp, shutting down and waiting, then greening back into work within minutes of the next shower. That knack of drying without dying is what I set out under how moss survives drought, and it is what lets a plant with no waterproof skin sit out a heatwave on open stone.
Getting the light right when you grow it
Which is to say there is no single answer to how much sun to give moss; you fit the moss to the light you have. For a shady, moist border or a north-facing bank, gather the soft carpet-formers of the woodland floor. For a sun-struck wall, roof or run of paving, turn instead to the tough cushion and wall mosses that expect to bake and rewet. Under glass the rule tips the other way: a terrarium wants bright but indirect light, and an hour of direct sun through the pane will cook the contents to straw, one of the commoner faults I list under terrarium troubleshooting. Set back from a window, moss does perfectly well on ordinary daylight or a cheap LED grow lamp, which is part of why it makes such an easy plant to keep indoors.
The short answer
So moss does need light, always; it just rarely needs direct sun, and it never needs none. Most kinds are happiest in the bright shade of a woodland edge, a few insist on open sun and a scorching stone, and not one will grow in a truly dark room. If you have been keeping a jar or a dish in a dim corner and puzzling over why the moss looks tired, move it somewhere brighter but out of the midday glare and give it a week or two. More often than not, light is the thing it was short of.