Sleep

Why 8 hours?
Only 2 hours sleep
17 hours of sleep!

Evolution to blame
Homo sapiens needs less sleep comparing to Apes (Hominoidea) because human beings use another strategy for surviving due to ability to walk on 2 legs

Insomnia
Causes of insomnia:
Physiological stress
Chronic pain
Hyperthyroidism
Heartburn,
Restless leg syndrome,
Menopause,
Certain medications, and drugs such as caffeine, nicotine, and alcohol
Other risk factors include working night shifts and sleep apnea.
However, there is no certain pathophysiological mechanism of insomnia
There are only some assumptions