Importance of the sun to lawn growth
The sun is of great importance to lawn growth because its rays reach the foliage and contribute to photosynthesis that produces carbohydrates useful to its survival.
A lawn that is constantly in the shade is continuously weak, sparse and overgrown with weeds. In other words, the daylight (without direct sunlight reaching the leaves) is not enough to provide good quality lawn.
In general, lawns need at least several hours of sunshine per day for satisfactory growth. The ideal would be to receive full sun all day. Even when using the shade seed (such as creeping red fescue), lawn growth will be better with a few hours of sunlight per day.
When your lawn is constantly in the shade, try to reduce the shade if possible. For example, trim some tree branches. If shade is caused by the presence of a wall and the lawn is continuously sparsely infected by powdery mildew disease and overgrown with moss and weeds, reduce irrigation in the shady spot, mow it higher and less frequently than the sunny spots.