There is now an international consensus that most adults need 7-9 hours' sleep per 24 hour period for optimal function and health.  Some adults can get away with 6 hours and some need as many as 10 hours, but sleep durations outside this range are not recommended.

There is a common belief that cutting back on sleep is a way to fit more into a busy lifestyle. The science is very clear that this is wrong.  Sleep is a time when the brain reconfigures itself to undergo a complex series of processes.  Many of these processes are unique to sleep and can only happen when the brain is no longer processing information from the senses (i.e., vision, sound, smell, taste).  There are two very different brain configurations during sleep: REM sleep when dreaming occurs and conversely, non-REM sleep, which includes light and deep sleep.

Some of the functions of sleep include:

• Memory and learning. During sleep, memories are stabilised and put into long-term storage, and the brain is primed for future learning.  So, the amount that you take away from the web content you read today will depend on both the amount and quality of sleep that you obtained last night and the amount and quality of sleep that you obtain tonight.
• Regulation of emotions. We make sense of our emotional experiences during sleep and insufficient sleep can make it difficult for us to regulate our emotions.  Sleep loss has also been linked to the generation of more negative and fewer positive emotions. 
• Repair of tissue wear-and-tear. Probably the function that is most commonly associated with sleep, repair of tissues primarily occurs during deep sleep.
• Hormone production. Some hormones (e.g., growth hormone, melatonin) are predominantly produced during sleep.
• Recharging the immune system. The immune system is re-primed for action during sleep and lack of sleep has been shown to substantially increase your risk of developing an infection.
• Regulation of appetite and metabolism. Sleep loss can alter hormones that influence appetite and satiety, as well as hunger and food choices.  Sleep loss also results in impaired glucose metabolism.