According to Matthew Walker's Why We Sleep, you should sleep at least seven hours every day, without interruptions, to let your brain go through the REM and NREM sleep phases necessary for the brain to develop properly, memories to be stored, and avoid long-term damage.
7–8 hours is within the safe zone, and anywhere above 9–10 hours isn't recommended.
World locations where daily afternoon naps are common seem to correlate to higher health and life expectancy levels. It seems that, in general, people who sleep well have to deal with fewer health problems and live longer.
This is a tiny summary, and I recommend reading the entire book. Sleeping and breathing are important parts of our life that we tend to know little about.