Key Difference: The word ‘Sleep’ is used as a noun/verb in a sentence and it refers to the action of sleep itself. The word ‘Asleep’ is used as an adverb/adjective in a sentence and it refers to the action of sleep as already in continuance or already completed.

For understanding the usage of word ‘Sleep’ let us consider the following examples:

• I like to sleep during Sunday afternoon.
• I went to sleep after watching the movie.
• I am going to sleep later.
• I can’t sleep because of the noise pollution.

Sleep is the state of unconsciousness that takes a natural course in every day life of all animals. The purpose of this state is to put the ‘body’ at rest in case of animals and put both ‘body and mind’ at rest in case of human beings.

When a person uses the word sleep in a sentence as observed in the above mentioned sentences, it means:

• The action of sleep is described as a verb/noun.
• The action can be described as generic activity,
• an activity already completed,
• an activity to be completed in near future
• Inability to complete the activity. 

For understanding the usage of word ‘asleep’ let us consider the following examples:

• Is your mother asleep?
• I wonder, if John is asleep?
• Please don’t call him now, I believe he is asleep!

• Sorry! I could not receive your call, I was asleep.

Being asleep means the action is already in progress or completed:

• The action of sleep is described as an adjective/adverb
• The action can be described as a specific activity,
• an activity already completed in the past
• an activity to be completed in near future as a prediction
• an activity in present continuous sense as in progress

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