What is a light gap you ask? When you have a blind that's internally mounted, there's a little strip of light shining through between the blind and the window frame. In the biz, we call this a light gap. You notice light gaps most when there is bright outdoor light and the room is made artificially dark by the blinds.
The pictures below illustrate a light gap, the same installation in different light situations.
In natural light, they are not bothersome to most people. Here is a Hartmann blind pulled part way up. Does the bright sliver of light between the blind and the molding bother you?
For some people, light gaps do not register. For others, they are unbearable. I am in the latter category -- The bottom picture produces a nearly blinding situation, a personal invitation for a migraine. I am not sure why. Maybe it's the high contrast of light to dark? I don't know...It's just like that. I see a light gap like this and I want to RUN FAR AWAY.
Since everyone's sleeping habits are so different, (and not everyone shares my special needs) it's worth thinking through what works and what doesn't. Some people need a room super dark. Others don't. The challenge is when bed partners have different needs. Me, I would sleep with nothing on the window if it weren't for commercial district next door. I don't mind day light. I sleep through it.
So if you are like me and hate a light gap but love a blind, here are some light gap minimizing tips.
1) Put panels over the window. A drape can be stationery and hide the dreaded light gap almost completely. The blinds can be the operable part. This is usually a more cost effective solution than operable drapes.
2) Stick to light filtering as opposed to room darkening products. Light filtering might offer a bit less darkness than opaque blinds so they create a less noticeable gap.
3) Try externally mounting your blinds. If the blind covers the molding the opportunity is not there for the light to creep around the blind. It's too far for the light to bend.