I have only been in a couple of HI rooms that had sheers only. It wasn't really a problem as I did not plan to sleep during the day and the windows did not face well-lighted areas (dark greenbelt IIRC). Blackout curtains are not standard equipment at all hotels, let alone all HIs. If you want to sleep during the day or feel there will be city lights, headlights, etc. shining into your window at night, switch rooms or check out.