With windows being mostly only at the front and at the back of the house, middle part can sometimes appear gloomy. You can follow those tips to bring more sunlight into your house.

1. Internal glass doors
They will cost a bit more than standard wooden ones, but they will allow sunlight to travel from one side of the building to another, making it brighter.

2. Interior windows
Adding an internal window will help to bright central parts of the house, as well as it will work as a design feature. 

3. Glass Balustrade
Changing your staircase balustrade for a glass one will also help to make the light flow between the different floor of the house. 

4. Glass steps
This will actually make the light travel through the different floors.

5. Skylight window
Skylight, where possible, will always add some extra light.

6. Adding a glass extension

In few words, add glass :)