Medicine

Poor sleeping habits (too little or too much) related to double risk of fatal cardiovascular disease (in humans). Recommended: 7 hours per night.
Shorter sleeping hours are related to Obesity, High Blood Pressure, and a higher risk of Type II Diabetes. Additionally, fatigue, tiredness and daylight sleepiness.
This study also found out that extending the sleeping hours to 8 hours a night also doubled the mortality in that group.
http://news.bbc.co.uk/2/hi/health/7009927.stm

A previous study (in animals) had found that lack of sleep caused the brain to stop producing new cells in the Hippocampus, via high levels of Corticosterone. When sleep was normalized in a week, brain cell production took two weeks to be restored, and it appears that the brain tried to boost neurogenesis in order to counteract the shortage.
http://news.bbc.co.uk/2/hi/health/6347043.stm