You can specify the brightness, color, location, and angle of "sun" lights and turn them on or off.
Change the Brightness of the Sun
- Click Render tab
Sun & Location panel

Sun Properties. - In General settings, click Intensity Factor and select a new value.
Turn Sun Lights On and Off
- Click Render tab
Sun & Location panel 

Sun Properties. - In General settings, click Status and select On or Off.