Friar Lawrence remarks upon Romeo's arrival at his cell at such an early hour.  The priest notes that "care," or worry is in the mind of an older person and can keep him awake at night as he ponders his concerns.  But, youth "with unstuffed brain," with the carefreeness of his age, can easily sleep for he has no worries to keep him awake:

But where unbruised youth with unstuffed brain/Doth couch his limbs, there golden sleep doth reign. (II,iii,31-32)

Because Romeo has thus arrived so early, Friar Lawrence deduces that either Romeo is ill or he has not been to bed at all.  He, of course, is correct as Romeo has spent much of the night talking with his beloved Juliet and promising to marry her.