Sleep needs vary substantially from birth (newborns typically need 16-18 hours of sleep), through childhood (10 to 12 hours per night), to adulthood (generally 7 to 8 hours per night) to old age (between 6-8 hours each night). In addition, in each age group there is considerable variation among individuals. Probably the most important indication that you are getting the amount of sleep you need is feeling refreshed without tiredness or sleepiness during the daytime.
If you experience a lull in energy or alertness in the early afternoon, or get sleepy when inactive (e.g., watching television), try sleeping an extra 30 to 60 minutes per night until your alertness and energy improves. Another sign that you may not be regularly meeting your need for sleep is sleeping more on weekends or vacations.