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Newborns typically sleep for about 16 to 18 hours a day, but the total amount of time spent sleeping gradually decreases to about 9 to 12 hours a day by age two years. At birth infants display a set of inherited involving such acts as sucking, blinking, grasping, and limb withdrawal. Infants’ vision improves from 20/800 (in Snellen notation) among two-week-olds to 20/70 vision in five-month-olds to 20/20 at five years. Even newborns are to certain visual patterns, chiefly movement and light-dark contrasts and show a noticeable preference for gazing at the human face; by the first or second month they can between different faces, and by the third they can identify their mother by sight. Young infants also show a for the tones of their mother’s voice, and they manifest a surprising sensitivity to the tones, rhythmic flow, and sounds that together make up human speech.