"Bright light" is the advice given for most melastomes, and it's not wrong exactly — but it papers over how differently light needs vary across this family depending on where a given genus originates. Applying Tibouchina's sun-loving habits to a shade-forest genus like Bertolonia is a fast way to scorch the leaves off a plant that was never built for it.
The full-sun end of the spectrum
Tibouchina, most Dissotis species, and Heterocentron all come from open, sunny habitats and want as much direct light as you can reasonably give them — full sun for at least six hours a day outdoors, or the brightest possible spot indoors, ideally supplemented with grow lights if a window alone won't provide enough intensity. Underlit plants in this group get leggy fast and flower poorly.
The bright-shade middle ground
Medinilla and most container-grown houseplant melastomes sit in the middle — they want bright, indirect light, similar to what an orchid or a philodendron prefers. Direct midday sun through glass can scorch them, but too little light produces weak growth and few flowers. An east-facing window, or a spot a couple of meters back from a bright south or west window, tends to work well.
The forest-floor end
Bertolonia and Sonerila come from the darker end of forest understory and genuinely dislike direct sun at any point in the day. These are the plants best suited to a terrarium or a shaded corner with supplemental grow lighting rather than a window at all — the goal is consistent, diffuse brightness rather than intensity.
The practical takeaway: before assuming a struggling melastome needs more light, check which part of this spectrum its genus actually belongs to. More light is only the right answer for about half the family.