Bright ..

In an office environment, the recommendation is for bright light. It means the iris of the eyes can be less dilated which gives a greater depth of field and thus places less strain on the eyes. This may be important for personnel approaching an age where they might need to start wearing glasses. You should consult standard HSE guidelines on this though, I'm not personally qualified in that area.

Low gloomy light will create depression leading to lack of efficiency.

Thanks @s. Sharma, @George Dishman and @Stanley Wilkin for your comments!