A few question about a curious phrase.

The curious phrase is: "...scared the living daylights out of me."

Just what are daylights?
If it's day, are such lights really needed?
Where do they go at night?
How do we know they're alive?
Why are they in people?
Why can they be scared out of people?
If a person has the living daylights scared out of them, do only dead daylights remain in them?
Are dead daylights of any use?
Is there a less traumatic means of performing a daylightectomy?
Is a person better off with or without daylights in them?