We know enough about what sleep is, though we don’t know what exactly causes sleep. Sleep is a state of rest — an unconscious rest. When we sleep, our body recovers from fatigue caused by the day’s activities. After a good sleep and the rest that it gives, we become alert and active again, ready for the normal activities of the day.
Of course, sleep needs no introduction. We know very well what sleep is. Sleep is a state of rest. During this state, our body and mind drift into a state of unconsciousness. We call this state an unconscious rest because, unlike the relaxation we take in between the day, we are almost unaware of what happens when we are asleep.
Sleep is a state of unconscious rest

When you are in a deep sleep, you may not realise that you have shifted your posture or have scratched your nose. You may not even realise that you've had a short power cut or that someone had crept in your and left while you were asleep. Of course, the latter part is quite creepy, but sleep is indeed an unconscious state where you are entirely oblivious to what is happening around us.

So, what causes sleep? Or why do we sleep? And, more importantly, why is it important to sleep?

To answer our first question, fatigue causes sleep. When we are tired, we tend to feel sleepy.
Fatigue causes sleep
But why do we sleep?  When we sleep, our body starts recovering from the tiredness caused by the day’s activities. We could recall feeling very tired in the evening, but imagine how we would feel after a good night's sleep? Very brisk and energetic, right? Or imagine that we never had enough sleep. How do you think we would feel? Drowsy, tired, and easily irritated.
Lack of sleep can make you feel tired and irritated

Lack of sleep can make us feel sluggish and lazy, blur our vision, disrupt our concentration, lose interest in work or studying, become less creative and productive, make more mistakes than usual, and so on. So sleep is essential to us. Lack of proper sleep (over a longer period) can make us look older than we are, help us gain unhealthy weight, cause us diseases, and may even lead to our death.

Hence, our body needs sleep to keep ourselves healthy, for the day and throughout our lives. After a good sleep, we become alert and active again. A night's worth of rest will make us ready for the normal activities of the day.
After a good sleep, we become alert and active again
Meanings of difficult words:
SleepThe resting state in which the body is not active and the mind is unconscious
RecoverTo become completely well again after an illness or injury
FatigueExtreme tiredness
AlertQuick to see, understand, and act in a particular situation
National Council of Educational Research and Training (2006). The Wonder Called Sleep. (pp. 25-27). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.