How much sleep do adults need? This is a very hard question to answer. The average person needs about seven to nine hours of sleep and sleeping less than sleep hours and more than nine hours are associated with increased risk of mortality. Now, many people tell me that they get five or six hours and they're fine, but I tell them if they're waking up with the use of an alarm clock, then it means that they're not getting enough sleep. So I tell people the amount of sleep that they need is the amount of sleep that they get where they don't feel sleepy the next day.
You should also be giving yourself and adequate amount of sleep throughout the seven nights out of the week. You cannot make up for sleep deprivation by sleeping in the next night. So for example, if you sleep five hours one night, you cannot make up for that lack of sleep by sleeping in ten hours the next night.
So sleeping less than six hours essentially means that you're sleep deprived. That puts you at risk for heart disease, it will lower your immune response to cold and other illnesses, it will also cause weight gain.
On the other hand, sleeping more than nine hours will also put you at risk for health problems, increasing obesity, which will also lead to heart disease. So that's why there's an increased risk in mortality when you sleep for more than nine hours and for less than six hours.
So just to stress, the average person needs about seven to nine hours of sleep and when you don't get that amount, that can have a multitude of heath problems and also affect your daytime functioning.