what makes you sleep at night? and why do we get tired? and good luck with the operation tomorrow!

Question: what makes you sleep at night? and why do we get tired? and good luck with the operation tomorrow!
- Keywords:
  good,
  luck,
  night,
  operation,
  sleep,
  tomorrow
Asked by ilth to Sarah on 25 Mar 2011.
- Sarah Thomas answered on 25 Mar 2011:
  
  Well scientists aren’t 100% sure about that! Some people say that it gives your body a chance to recuperate after a hard day, but the energy saved is only about 50 kcals for 8 hours sleep (about the same amount of energy in a slice of toast with no butter!). I think there might be an evolutionary reason, traditionally humans couldn’t do much at night because we don’t have good night vision compared to lots of predators!
  
  Also, think about what would happen if we didn’t sleep. Lack of sleep has serious effects on our brain’s ability to function. Have you ever pulled an all-nighter?? and the after-effects: grumpiness, grogginess, irritability and forgetfulness. After just one night without sleep, concentration becomes more difficult and attention span shortens considerably.
  
  Thanks for the good luck – the operation went really well!! But I am pretty sore and groggy today! The pain killers and anesthetic are not making me feel good! I’ve now got to wear a cast for 4 weeks – booooo!!!

Question: what makes you sleep at night? and why do we get tired? and good luck with the operation tomorrow!