Can I use the sun as a light source when filming?
Every light source has a CRI rating, which quantifies how accurately colors are reproduced.
A CRI score of 95 or higher indicates good video production lighting. Yes, daylight is an excellent source of light since the sun has a CRI of 100, which is the reference against which all other lights are measured.
If you don't have access to any light equipment, illuminating your scene using a window is a fantastic substitute. Set your camera's white balance to daylight or 5000 to 5600K, and make sure your subject is facing the window so that the window serves as a key light.
The disadvantage of using natural light as your main light is that you have no control over it. The light cannot be controlled, but it may be softened or distributed wider by reflecting it.
The issue that might occur is that the scene's lighting suddenly will start to change. If the sun was first shining, one shot may be really well lit, but the next shot might be dull if the sun fell behind a cloud.
Because you are unable to control the light, as a filmmaker or video producer it is better to draw the curtains and cover up the windows to keep the sun from affecting your scene.
The sun may be a great source of light when needed, but it's usually best to restrict its impact on a scene so that you can have complete control over the lighting situation.