I thought having window blinds open for takeoff and landing was to allow emergency service staff to be able to look inside (e.g., to see if there's fire in the cabin).

I have tried the sleeper service on BA on short trans-Atlantic services and had to put up with the noise of cabin announcements as well as a brightly lit cabin for quite some time after departure. I found this made sleep difficult. The activity was due to some extent to some opting to eat onboard plus all the usual (some quite unnecessary) announcements. This was some time ago so perhaps it's different now.