I have a late 2010 11-inch MBA running 10.9.3. It used to wake up from sleep immediately after opening the lid. But starting very recently, often when I open the lid the screen remains black. To wake the screen I also have to press the space bar. This happens (almost) always if the computer has not been used for several hours. If it was asleep for not too long, opening the lid is enough. Have they changed something recently? Is it a normal behaviour or something went wrong with my computer's hardware or software?