RHYMING RIDDLES

I'm in the morning, and evening too,
Though not in afternoon;
You may search the sky - I'm not there
Not in earth, nor sun, nor moon.
Yet you may plainly perceive me,
For like a balloon, I am midway suspended in air.

Though disease may possess me,
And sickness and pain,
I'm never in sorrow nor gloom.
Though in wit and wisdom I equally reign,
I'm the heart of sin and have long lived in vain.
Always in life, I shall never be found in the tomb.

Answer:

The letter "I".

Credit:
Lord Byron 
(adapted)