Sleep is a common noun because it refers to a general concept or idea rather than a specific person, place, or thing. Common nouns are not capitalized unless they appear at the beginning of a sentence. In the case of sleep, it is a regular activity that all people engage in, making it a common experience shared by many.

Why sleep is a common noun?
- General concept: Sleep is a universal phenomenon that encompasses the act of resting in a state of unconsciousness. It is not specific to any particular individual or location, which qualifies it as a common noun.
- Shared experience: Everyone needs to sleep to maintain their physical and mental well-being. It is a routine part of human life that is not unique to any one person, making it a common occurrence among all individuals.
- Not a proper noun: Proper nouns refer to specific names or titles, such as names of people, places, or organizations. Since "sleep" does not refer to a particular entity, it falls under the category of common nouns.
- Capitalization: Common nouns like sleep are typically written in lowercase letters, unless they are used at the beginning of a sentence. This distinguishes them from proper nouns, which are always capitalized.
In conclusion, sleep is considered a common noun because it represents a general concept that is shared by all individuals. It is a fundamental aspect of human existence that is not unique to any one person, place, or thing.