If you are an avid fan of action films and classics like Batman, you might be inclined to try and look for facial features to identify the silhouette that approaches you in the dark.
But you really should be looking at your opponent’s head level (not at the eyes trying to see them flinch,) The idea is that you do not want the person's both arms and legs escape your vision.
At the same time you need to look straight parallel to your belt where you kicks are coming from or parallel to your shoulders where your hand strikes are coming from and use your vision to measure the distance of your strikes and defenses. At split seconds your vision might shift during defense and attack maneuvers since you will be re-positioning yourself and would not be able to have a complete view of your opponent.
A silhouette is approaching at the dark, is it an instinct to try and identify the face, or perceive it as a threat and learn to be alert? Are we going to look for the arms for weapons? Are we going to wait for the point that even if we notice a surprise weapon it would be too late for us to react?
Our eyes serve as distance gauge and a tool for perception of the predicted scenario. We need to see what to expect, we need to see what we catch, stop, or where we aim. We can never see our own hands in complete motion because it would slow it down. Do you think a golf player or a batter can see his complete swing at the same time he is swinging?
So you basically try to keep your head facing centered to where the opponent is, keep your eyes focused on the center, on the vertical line of the opponent's body at your eye level.
Look at eye level, because it is parallel to your waist where the kicks are coming from so you measure the exact distance to lunge and reach him. If your opponent is still in a safe far range, you have the time to peel into other areas and see if he is holding a weapon or look for any other details you may wish.
In the closer range where hands are more effective and efficient, there is no discrepancy, and in the longer range where the legs pose the danger, you can still see the feet in your peripheral vision.
Nothing then is dangerous above or below since if the opponent ducks he has all his weight on his front foot and cannot kick. At the same time there is nothing above the arms that can attack you either.