Every Hindus are instructed to Rub the palms, warm up eyes and then open the eyes slowly when we wake up in the morning. This is because Eyes have two types of photoreceptors - Rods and Cones. Rods detect the light where as cones distinguish the colors - red, blue and green. Cones are non functional in the dark and hence in the night if we are in sudden dark our eyes strain out and even same when we are out of sudden dark. This is because it takes a fraction of minute for the cones to become active. Similarly when we are asleep these photoreceptors are in a rested state, if we suddenly open eyes they will be strained out.