For adults, it is between 7-9 hours. However, as we age, we tend to need fewer hours of sleep, which can be as less as six. The challenge is the more you read about sleep, the more you start checking your sleep hours and just the knowledge that the amount you sleep is perhaps not within prescribed standards, even if you sleep well, can lead to sleep problems, where none earlier existed. It is a good idea to keep regular sleep hours and if the quality of your sleep is good, not to worry too much about whether you sleep for 6 1/2 or 7 or 8 hours. Typically however, a sleep duration of less than 6 hours is considered to be harmful.
This probably matters more than the length of sleep. If you sleep for 8 hours but are constantly disturbed, or keep getting up, or don’t get enough deep sleep, you may be worse off than someone who sleeps for 6 hours, but has undisturbed deep sleep. How do you know that you have had good sleep? Typically, the way you feel in the morning and throughout the day will help you understand whether you are sleeping well or not. If you constantly feel drowsy and sleepy during the day, it generally means that you are either sleeping less or the quality of your sleep is poor or both.