This question always intrigues me, because it often serves as a barometer for how society looks at the value of sleep. By way of analogy, we can look at how we count currency to show us why counting hours of sleep pales in comparison to measuring the quality of your slumber. If someone offered you a gift of 100 dollars or 100 gold coins, I trust you would accept the latter, knowing their worth would exceed by 1,000 times what you could cash in with the former. Just so, the number of hours of sleep you obtain is of much less value to your health than the number of hours of high quality sleep you receive.
If you sleep 5 hours per night, wake up refreshed, and can tap into abundant energy throughout the day without artificially stimulating yourself with caffeine, then apparently 5 hours of sleep is right for you. In contrast, if you sleep 10 hours per night, then struggle to wake up, and pray for an IV infusion of caffeine to help you crawl out of bed, your slumber is probably broken, and no amount of sleep hours will solve such a problem.
When encountering patients at the sleep center perplexed by this question, we want them to appreciate as rapidly as possible that no one could ever determine the right number of hours of sleep until they successfully treated their sleep problems to restore a state of normal or near normal sleep quality.
Along these lines, it is an illuminating paradox in the field of sleep medicine that someone with classic sleep apnea (sleepiness-predominant symptoms) might sleep 10 hours per night and feel rotten the next day, and someone with sleep apnea-driven insomnia (sleeplessness-predominant symptoms) might sleep 5 hours per night and also feel rotten the next day. In both cases, sleep apnea destroyed the sleep quality in that individual, but in one case the patient presents with hypersomnia (sleeps too much), and in the other case, the patient presents with insomnia (sleeps too little). Yet, in both cases the underlying sleep quality problem was the same fragmenting effects induced by disruptive sleep breathing patterns that trigger arousals and awakenings all night long.
Ironically or not, when both these types of patients receive treatment, the hypersomnia patient could easily drop total sleep time to 7.5 hours per night using PAP therapy; and the insomnia patient might increase total sleep to 7.5 hours using the combination of PAP therapy and CBT-I.
Summing up, quality counts! Finding a way to measure the quality of sleep is of far greater importance to the patient than narrowly focusing on the superficial metric of sleep quantity. While there are always exceptions to this approach, it is remarkable how often addressing sleep quality problems first leads to the resolution of sleep quantity problems.