I, too, find it easier to do Qigong practice during the prescribed hours. When I am in a group, however, any time is just as easy. It's got to be the group energy.
Sure, doing those exercises before bed would still be beneficial.
An advantage of doing them just before studying is that you open the energy channels making it easier for the brain/body to do its thing. Maybe try two minutes worth of exercise and see if that helps. I'm sure it would, and I think you would then find it easier to creep up to three minutes, then five minutes...