The amount of sleep needed varies greatly from individual to individual.There are people who need 9-10 hours of sleep every night and people who only sleep 3-4 hours.
There are also people who don't sleep at all (well, at least sometimes they don't)...that's because they can't!

I believe that's what they call insmonia

Insomnia can take various forms: difficulty falling asleep (sleep onset insomnia), difficulty staying asleep, and early wakening. Sleeplessness is common during pregnancy, especially in the later weeks. The elderly typically sleep lightly and fitfully.

Physical situations can complicate sleep patterns: the need to urinate frequently, leg cramps, painful conditions such as arthritis, symptomatic asthma or other medical problems and restless legs syndrome can all interfere with both sleep onset and sleep maintenance. Social situations can disturb sleep, too. Arguing, watching exciting programs on television late at night, consuming excessive caffeine or alcohol or eating a large meal close to bedtime, vigorous exercise less than six hours before retiring may disturb sleep patterns.

In the majority of insomnia cases, however, the core problem is emotional. Anxiety and internalized, unexpressed anger are common causes of sleeplessness. Depression is also implicated in insomnia, waking in the early morning is common in some depressed individuals. People may clock many hours of sleep time, yet continue to feel fatigued due to poor sleep quality.

Paradoxically, insomnia may result from the use of a sedative prescribed to relieve it. Some people, especially the elderly or people with certain occupations, develop an inverted sleep rhythm: drowsiness in the morning, sleep during the day, and wakefulness at night.