For most people, three hours three times a day is not as good as six hours straight. Our sleep runs in cycles, and the REM sleep cycle increases the longer we are asleep. The first time you enter REM it doesn't last very long. You don't get nearly as much REM sleep in 3 hour naps as you would if you get to that 2nd and 3rd cycle.
It's not how long you sleep it's how well you sleep. Sleep is broadly divided in the REM and NREM (non-REM) phases. NREM sleep is further divided into N1, N2, and N3. The normal sleep cycle is N1 → N2 → N3 → N2 → REM. This cycle repeats several times while you are sleeping. If at any point in your sleep you wake up, even for a split second, you have to start over at N1. Stage N1 refers to the transition of the brain from alpha waves having a frequency of 8–13 Hz (common in the awake state) to theta waves having a frequency of 4–7 Hz. Stage N2 is characterized by sleep spindles ranging from 11–16 Hz (most commonly 12–14 Hz) and K-complexes. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. Stage N3 (deep or slow-wave sleep) is characterized by the presence of a minimum of 20% delta waves ranging from 0.5–2 Hz and having a peak-to-peak amplitude >75 μV. (EEG standards define delta waves to be from 0–4 Hz, but sleep standards in both the original R&K, as well as the new 2007 AASM guidelines have a range of 0.5–2 Hz.) This is the stage in which parasomnias such as night terrors, nocturnal enuresis, sleepwalking, and somniloquy occur. N3 sleep is a time when the body repairs itself and builds up energy for the day ahead. It plays a major role in maintaining your health, stimulating growth and development, repairing muscles and tissues, and boosting your immune system. In order to wake up energized and refreshed, getting quality deep sleep is key. So even if you sleep for 9 hours, if your N3 sleep is insufficient you will not feel rested. Your internal body clock can also play a part. Internal clocks can vary from a 23 hour day to a 25 hour day. It can sometimes be difficult for people with an internal clock of other than a 24 hour day to maintain regular sleeping patterns. Sleep needs vary with each individual. If you think you have a sleep problem you should consult your doctor about it.
I think 8 hours or minimally, 6 hours. Unfortunately, I don't enjoy such privilege due the nature of my work. At times an hour sleep is enough for me.
It really depends on person to person but something between 5 - 8 hours is needed for a normal adult.
4 hours of sleep is enough when you wake up in the morning feeling good...Babies needs more sleep than adults so, adults sleep of just 4 hours is enough for them.
It depends on your age. But normally you have to sleep at least 8 hours a day.
I think 7 hours is about average/"normal".
It varies with age but the more the better for me. I get cranky without sleep.
I've heard that Einstein only slept 3 or 4 hours a night., but I think that's the exception. Most doctors say 8 hours is optimal, but I think it depends on the person. Some do well on 6, some need 9. And from what I've read, it's best to get it all at one time.