Why do our eyes allow us to sleep easier in the dark than light?

Question: Why do our eyes allow us to sleep easier in the dark than light?
- Keywords:
  eye,
  light,
  sleep,
  vision
Asked by Anyadb to Anna, Hayley, Iain, Rebecca on 16 Jun 2015. This question was also asked by guineapigs&me.
- Rebecca Jones answered on 16 Jun 2015:
  
  This dates back to the stone age when artificial lights we use today weren’t around. Back then, humans had two types of natural light which regulated our circadian rhythm (changes that follow a 24hour cycle). These were the sun and then the moon and stars. As day and night were so well defined our biological systems followed suit, allowing us to sleep during the night and be awake during the day

Question: Why do our eyes allow us to sleep easier in the dark than light?