For most people, getting 8 to 9 hours of sleep increases concentration, improves physical 6, and improves one's mood.

It is not true when they say that when you are older you need less sleep. Adults like children still need 7 to 8 or even 8 to 9 hours of complete sleep in order to feel rejuvenated for the next day. Getting enough sleep helps the body to recover and to heal itself well in preparation for the next challenging activities it needs to face. Studies have shown that people who complete a sleep cycle tend to be more happy, fulfilled, and alive.   

What happens to the influenza virus every 20-40 years and why?