Here's why:
* Two independent clauses: The sentence contains two complete thoughts: "You are tired" and "need to sleep today."
* No conjunction or punctuation: There is no conjunction (like "and", "but", "or") or punctuation (like a semicolon or period) to separate these two clauses.
To correct it, you could:
* Add a conjunction: "You are tired and need to sleep today."
* Use a period: "You are tired. Need to sleep today."
* Use a semicolon: "You are tired; need to sleep today."