My left eyelid is slightly lower than the right, and is worse on a morning. why could this be?

Ptosis: Eyelid drooping, which is also called ptosis, may occur due to trauma, age, or various medical disorders. This condition can affect one or both eyes, it may come and go, or it might be permanent. Depending on the severity of the condition, drooping eyelids can block vision. In most cases, however, the condition is treatable, often through surgery. See an Ophthalmologist for an evaluation.

Part is: Just the way you are built. None of us are symmetrical . When you are tired the differences are usually more easily noticed.