Why do we sleep?

There are actually 2 factors that makes us sleep. 
1. Light
2. Adenosine

1.Light:
When we see light in the morning, our body releases more amount of Cortisol which keeps us awake. While when it is dark, our body releases more amount of melatonin which makes us sleep.

2.Adenosine :
Accumulation of adenosine is the second factor that makes us sleep. Adenosine is accumulated in our brain because of stress, work loads during day time. Now, when we sleep, our brain take time reabsorb the accumulated adenosine. So that our brain is ready for the next day.
                Besides this, sleeping helps brain to reorganize information and clear unwanted memories. Moreover, while sleeping we use very less energy. Thus, sleeping also helps us to conserve energy.