I found something that addresses both questions
"The amount of sleep we need varies from person to person, and depends on many factors, including age. Infants tend to need the most sleep, averaging about 16 hours a day, while teenagers need about 9 hours. Most adults, on the other hand, need about 7 to 9 hours of sleep to feel fully rested. . ."

And why
" Beyond just restoring your body's energy levels, there are many benefits of sleep. At the most basic level, studies have shown that sleep is necessary for survival. You also need good sleep so that you can function well–both mentally and physically–during your waking hours. Good sleep can enable you to work productively, make sound judgments, avoid harm, recover from illness, and interact with other people effectively. "
Source(s):
http://www.shuteye.com/stages-of-sleep/benefits-of-sleep.aspx

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I've always wanted to try a Polyphasic sleep schedule. A couple of sites that I have read reference people like DaVinci utilizing this type of sleep schedule. My very high level understanding is that once you pick your schedule, you HAVE to stick to your schedule.

Find out more here:

http://en.wikipedia.org/wiki/Polyphasic_sleep

http://www.stevepavlina.com/blog/2005/10/polyphasic-sleep/

http://www.supermemo.com/articles/polyphasic.htm
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Do you feel drowsy? Can't stop yawning? Maybe you don't get as much sleep as you should. Sleep is important; even one night of inadequate sleep can impair your reaction time and mental functioning, and chronic sleep deprivation can negatively impact your immune system and contribute to the development of a whole host of serious ailments. Here are some tips to help you determing how many hours of sleep you need:
1. Determine a time when you will wake up every morning. (set your alarm for that time. If possible, set alarms to wake up earlier then that time to gradually awaken you.)
2. Abstain from alcohol, caffeiene, cigarettes, and medicines that can affect your sleep.
3. Avoid taking naps. (if you need to take a cat nap of 10-15 minutes, thats fine, but otherwise do not take long naps.)
4. Adjust your bedtime as necessary to compensate for new wake-up times.

i hope this helps :)
Source(s):
www.wikihow.com

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Quantity and quality are very important. Most adults need between 7.5 to 8.5 hours of uninterrupted sleep. If you press the snooze button on the alarm in the morning you are not getting enough sleep. This could be due to not enough time in bed, external disturbances, or a sleep disorder.

http://www.sleepnet.com/tips.html

http://t1.gstatic.com/images?q=tbn:YUXtHvm4jPb4FM:http://featured.geoparent.com/wp-content/uploads/2008/07/woman-sleeping.JPG
Source(s):
http://t1.gstatic.com/images?q=tbn:YUXtHvm4jPb4FM:http://featured.geoparent...

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9-10 hours of sleep is what is needed in a day.For adults,for children it is a little more than that.
Source(s):
http://www.callcarenet.com

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There's no magical number of hours required. We don't need any set of amount of sleep - it's all down to genes and whether we feel fresh and alert during the day. Prof Horne , the author of Sleepfaring .says: "The test of insufficient sleep is whether you are sleepy in the day or if you remain alert through most of the day."

Prof Horne, says : A journey through the science of sleep, says the idea that Victorians got lots of sleep and had a better work-life balance is a myth.

If you sleep for eight hours a night go to work and find yourself lolling and drooling on the keyboard, you aren't getting enough. If you're sleeping five hours and running the country, you probably are getting enough.

The US National Sleep Foundation suggests seven to nine hours a night is advisable for adults, and a survey it conducted in 2002 suggested three-quarters of Americans had problems sleeping and a third were so sleepy during the day their activities were affected.
Source(s):
US National Sleep Foundation GUIDE LINES

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