Sleep

Here is an interesting article about biphasic sleep (two sleep session per day vs. monophasic - one large sleep block).

Sleep is uber important for a few reasons, the how part is not totally understood so don't get hung up on that part.  Just know that it's important for:

Generally, lack of sleep may result in^[3][4]

• aching muscles^[5]
• dizziness and nausea
• dry mouth^{[citation needed]}
• hallucinations^[6]
• hand tremors^[7]
• headaches
• increased blood pressure^[8]
• increased risk for diabetes^[9]
• increased risk of fibromyalgia^[10]
• irritability^[3]
• memory lapses or loss^[11]
• nystagmus (rapid involuntary rhythmic eye movement)^[12]
• obesity^[9]
• slowed word recall^{[citation needed]}
• temper tantrums in children^[3]
• yawning^[3]
• symptoms similar to:
  
  
  • Attention-deficit hyperactivity disorder^[3] (ADHD)
  • psychosis^[13]

From Wikipedia

For more about sleep deprivation click here
Or check out this site Sleep Depreivation
Our National Govt. Recommendations
Why we sleep
An alternative approach