We've moved into a house with windows with a recess so a windowsill of about 10 cm/, and UPVC frames i.e. with angles that edge onto the glass. In our old place we had wooden sashes that were very narrow so no problems but now... There's a streetlight right opposite the DD's bedroom, so even in winter it's pretty bright, and DD2 in particular is very sensitive to the light.
We got special blackout curtains made, as the window in the DD's bedroom is nearly 2m wide so too big for normal ones. Installed these, the light still floods up the gap where the curtain rail is attached to the wall. We tried fixing a Gro-blind to the window, but the suckers won't stick to the frame, and when we stick it on the glass even with adjusting it, there are still gaps where the light comes through, then up by the curtain rail. So I bought a blackout roller blind. Bloody thing of course also sits about a cm away from the wall/frame so the light still gets through.
Apart from fitting a pelmet (urgh) and velcro-ing the curtains to the walls, is there any other way to actually stop light getting into the room?! DD2 has been waking 5 minutes earlier every morning for the last 2 weeks as the sunrise gets earlier and I can't take much more...